Add requrements/ dir containing abstract *.in and concrete *.txt

... parametrized by python version.

We have a bunch of dependencies that are only needed on specific python
versions (e.g. backports like dataclasses, importlib_resources, etc).
We use conditional environment markers for those in the top-level
requirements.in files.
pip-compile does not yet support generating a combined requirements.txt
file where differences across python versions/platforms/archs are
reconciled using environmet markers:
jazzband/pip-tools#826

They currently recommend running pip-compile on each targeted python
environment generating as many concrete requirements.txt files
jazzband/pip-tools#651

So this is what I have done here.

requirements/dev-requirements.in

@@ -0,0 +1,9 @@
+# This file only declares the top-level development requirements.
+# Run `pip-compile-all.sh` to generate the concrete py3*-requirements.txt
+# files that are used to set up the tox and CI environments.
+
+black
+pytest
+# As of November 2020, pytype requires: Python <3.9, >=3.6
+# https://pypi.org/project/pytype/2020.11.23
+pytype; python_version < '3.9'

requirements/install-requirements.in

@@ -0,0 +1,6 @@
+# Only list here top-level runtime dependencies.
+# Run `pip-compile-all.sh` to generate the concrete py3*-requirements.txt
+# files that are used to set up the tox and CI environments.
+dataclasses>=0.7; python_version < '3.7'
+lxml>=4.0
+skia-pathops>=0.4.1

requirements/pythoh3.7-requirements.txt

@@ -0,0 +1,33 @@
+#
+# This file is autogenerated by pip-compile
+# To update, run:
+#
+#    pip-compile --output-file=requirements/py37-requirements.txt requirements/dev-requirements.in requirements/install-requirements.in
+#
+appdirs==1.4.4            # via black
+attrs==20.3.0             # via pytest, pytype
+black==20.8b1             # via -r requirements/dev-requirements.in
+click==7.1.2              # via black
+decorator==4.4.2          # via networkx
+importlab==0.5.1          # via pytype
+importlib-metadata==3.1.0  # via pluggy, pytest
+iniconfig==1.1.1          # via pytest
+lxml==4.6.2               # via -r requirements/install-requirements.in
+mypy-extensions==0.4.3    # via black
+networkx==2.5             # via importlab
+ninja==1.10.0.post2       # via pytype
+packaging==20.4           # via pytest
+pathspec==0.8.1           # via black
+pluggy==0.13.1            # via pytest
+py==1.9.0                 # via pytest
+pyparsing==2.4.7          # via packaging
+pytest==6.1.2             # via -r requirements/dev-requirements.in
+pytype==2020.11.23 ; python_version < "3.9"  # via -r requirements/dev-requirements.in
+pyyaml==5.3.1             # via pytype
+regex==2020.11.13         # via black
+six==1.15.0               # via importlab, packaging, pytype
+skia-pathops==0.5.1.post1  # via -r requirements/install-requirements.in
+toml==0.10.2              # via black, pytest
+typed-ast==1.4.1          # via black, pytype
+typing-extensions==3.7.4.3  # via black
+zipp==3.4.0               # via importlib-metadata

requirements/python3.6-requirements.txt

@@ -0,0 +1,34 @@
+#
+# This file is autogenerated by pip-compile
+# To update, run:
+#
+#    pip-compile --output-file=requirements/python3.6-requirements.txt requirements/dev-requirements.in requirements/install-requirements.in
+#
+appdirs==1.4.4            # via black
+attrs==20.3.0             # via pytest, pytype
+black==20.8b1             # via -r requirements/dev-requirements.in
+click==7.1.2              # via black
+dataclasses==0.8 ; python_version < "3.7"  # via -r requirements/install-requirements.in, black
+decorator==4.4.2          # via networkx
+importlab==0.5.1          # via pytype
+importlib-metadata==3.1.0  # via pluggy, pytest
+iniconfig==1.1.1          # via pytest
+lxml==4.6.2               # via -r requirements/install-requirements.in
+mypy-extensions==0.4.3    # via black
+networkx==2.5             # via importlab
+ninja==1.10.0.post2       # via pytype
+packaging==20.4           # via pytest
+pathspec==0.8.1           # via black
+pluggy==0.13.1            # via pytest
+py==1.9.0                 # via pytest
+pyparsing==2.4.7          # via packaging
+pytest==6.1.2             # via -r requirements/dev-requirements.in
+pytype==2020.11.23 ; python_version < "3.9"  # via -r requirements/dev-requirements.in
+pyyaml==5.3.1             # via pytype
+regex==2020.11.13         # via black
+six==1.15.0               # via importlab, packaging, pytype
+skia-pathops==0.5.1.post1  # via -r requirements/install-requirements.in
+toml==0.10.2              # via black, pytest
+typed-ast==1.4.1          # via black, pytype
+typing-extensions==3.7.4.3  # via black
+zipp==3.4.0               # via importlib-metadata

requirements/python3.7-requirements.txt

@@ -0,0 +1,33 @@
+#
+# This file is autogenerated by pip-compile
+# To update, run:
+#
+#    pip-compile --output-file=requirements/python3.7-requirements.txt requirements/dev-requirements.in requirements/install-requirements.in
+#
+appdirs==1.4.4            # via black
+attrs==20.3.0             # via pytest, pytype
+black==20.8b1             # via -r requirements/dev-requirements.in
+click==7.1.2              # via black
+decorator==4.4.2          # via networkx
+importlab==0.5.1          # via pytype
+importlib-metadata==3.1.0  # via pluggy, pytest
+iniconfig==1.1.1          # via pytest
+lxml==4.6.2               # via -r requirements/install-requirements.in
+mypy-extensions==0.4.3    # via black
+networkx==2.5             # via importlab
+ninja==1.10.0.post2       # via pytype
+packaging==20.4           # via pytest
+pathspec==0.8.1           # via black
+pluggy==0.13.1            # via pytest
+py==1.9.0                 # via pytest
+pyparsing==2.4.7          # via packaging
+pytest==6.1.2             # via -r requirements/dev-requirements.in
+pytype==2020.11.23 ; python_version < "3.9"  # via -r requirements/dev-requirements.in
+pyyaml==5.3.1             # via pytype
+regex==2020.11.13         # via black
+six==1.15.0               # via importlab, packaging, pytype
+skia-pathops==0.5.1.post1  # via -r requirements/install-requirements.in
+toml==0.10.2              # via black, pytest
+typed-ast==1.4.1          # via black, pytype
+typing-extensions==3.7.4.3  # via black
+zipp==3.4.0               # via importlib-metadata

requirements/python3.8-requirements.txt

@@ -0,0 +1,31 @@
+#
+# This file is autogenerated by pip-compile
+# To update, run:
+#
+#    pip-compile --output-file=requirements/python3.8-requirements.txt requirements/dev-requirements.in requirements/install-requirements.in
+#
+appdirs==1.4.4            # via black
+attrs==20.3.0             # via pytest, pytype
+black==20.8b1             # via -r requirements/dev-requirements.in
+click==7.1.2              # via black
+decorator==4.4.2          # via networkx
+importlab==0.5.1          # via pytype
+iniconfig==1.1.1          # via pytest
+lxml==4.6.2               # via -r requirements/install-requirements.in
+mypy-extensions==0.4.3    # via black
+networkx==2.5             # via importlab
+ninja==1.10.0.post2       # via pytype
+packaging==20.4           # via pytest
+pathspec==0.8.1           # via black
+pluggy==0.13.1            # via pytest
+py==1.9.0                 # via pytest
+pyparsing==2.4.7          # via packaging
+pytest==6.1.2             # via -r requirements/dev-requirements.in
+pytype==2020.11.23 ; python_version < "3.9"  # via -r requirements/dev-requirements.in
+pyyaml==5.3.1             # via pytype
+regex==2020.11.13         # via black
+six==1.15.0               # via importlab, packaging, pytype
+skia-pathops==0.5.1.post1  # via -r requirements/install-requirements.in
+toml==0.10.2              # via black, pytest
+typed-ast==1.4.1          # via black, pytype
+typing-extensions==3.7.4.3  # via black

requirements/python3.9-requirements.txt

@@ -0,0 +1,25 @@
+#
+# This file is autogenerated by pip-compile
+# To update, run:
+#
+#    pip-compile --output-file=requirements/python3.9-requirements.txt requirements/dev-requirements.in requirements/install-requirements.in
+#
+appdirs==1.4.4            # via black
+attrs==20.3.0             # via pytest
+black==20.8b1             # via -r requirements/dev-requirements.in
+click==7.1.2              # via black
+iniconfig==1.1.1          # via pytest
+lxml==4.6.2               # via -r requirements/install-requirements.in
+mypy-extensions==0.4.3    # via black
+packaging==20.4           # via pytest
+pathspec==0.8.1           # via black
+pluggy==0.13.1            # via pytest
+py==1.9.0                 # via pytest
+pyparsing==2.4.7          # via packaging
+pytest==6.1.2             # via -r requirements/dev-requirements.in
+regex==2020.11.13         # via black
+six==1.15.0               # via packaging
+skia-pathops==0.5.1.post1  # via -r requirements/install-requirements.in
+toml==0.10.2              # via black, pytest
+typed-ast==1.4.1          # via black
+typing-extensions==3.7.4.3  # via black