Install package locally

Prior to this change, the `timezone_tools` package was not installed
locally and so you had to pass a `PYTHONPATH` environment variable to
pytest to run tests.

This isn't a problem when running tests in `tox` and it automatically
installs the first-party package as part of its set-up phase.

This change updates the `makefile` to install the `timezone_tools` in
editable mode.

Now it's possible to run `pytest` from repo root.

CONTRIBUTING.md

@@ -25,9 +25,9 @@ source venv/bin/activate
 You could also use [virtualenvwrapper], [direnv] or any similar tool
 to help manage your virtual environments.
 
-### Install the Python dependencies
+### Install the `timezone_tools` package and its dependencies
 
-To install all the development dependencies in your virtual environment,
+To install the `timezone_tools` package and all the development dependencies in your virtual environment,
 run:
 
 ```sh
@@ -37,8 +37,6 @@ make dev
 [direnv]: https://direnv.net
 [virtualenvwrapper]: https://virtualenvwrapper.readthedocs.io/
 
-The tools used to manage this project will be installed.
-
 ### Run the tests
 
 To start the tests with [tox](https://tox.wiki), run:
@@ -48,10 +46,10 @@ tox
 ```
 
 Alternatively, if you want to run the tests directly in your virtual environment,
-you many run the tests with:
+you may run the tests with:
 
 ```sh
-PYTHONPATH=src python3 -m pytest
+python3 -m pytest
 ```
 
 ### Static analysis

makefile

@@ -7,4 +7,5 @@ dev: install_python_packages
 
 .PHONY:install_python_packages
 install_python_packages: requirements.txt
-	python3 -m pip install -r requirements.txt
+	# Install the `timezone_tools` package in editable mode.
+	python3 -m pip install -r requirements.txt -e .