Refactor package template to follow APE 17

[astrofrog]

👷‍♂️ This refactors the package template to follow the recommendations in APE 17. Specifically, the changes are:

• Added the APE 17 migration guide to the docs
• Removed anything to do with astropy-helpers
• Added a pyproject.toml file
• Updated setup.py to match what we have in astropy core
• Added a tox.ini configuration file
• Updated Travis CI configuration to use tox
• Make use of setuptools-scm
• Removed coveragerc in favor of having the coverage config in setup.cfg
• Updated conftest.py

In addition I have made the following changes:

• Added the use_compiled_extensions option for cookiecutter, because now there are some changes that are required for compiled extensions even if there is no example code.
• Removed include_example_cython_code and instead rely on use_compiled_extensions used together with include_example_code
• Removed the long_description question in favor of using the README.rst as is often done
• Removed the automodapi templates which should no longer be needed (not sure why we still had them)

📖 To see a preview of the migration guide, see https://external-builds.readthedocs.io/html/astropy-package-template/438/index.html (or click 'Details' in the status checks for the RTD build)

⚠️ If you want to try out the guide, in Step 0, use the following to use the template from this PR:

cookiecutter gh:astrofrog/package-template --checkout infrastructure-refactor -o my_package_tmp

There is still some cleanup I'd like to make to the template unrelated to APE 17, so I'll leave this to another PR. In particular, @Cadair has been doing some work creating a more generic non-astropy-specific template and as part of that has developed a nice way to test the template, but that can wait for another PR.

Preview of the rendered template: https://github.com/astrofrog/package-template/tree/preview-ape17

[astrofrog]

This is now ready for review!

[astrofrog]

Ok I've now simplified the migration guide a bit to avoid repeating whole files when the file can just be copied over as-is from the generated template without any modifications.

[astrofrog]

@pllim @mhvk @bsipocz - I've pushed a few more changes:

• Fixed edit_on_github to always use master
• Fixed the version specification for the current package in conftest.py
• Made pytest-astropy-header optional
• Added a note about why conftest.py has to live in the source tree for now
• Added a note about updating conda-forge recipes

For the issue of tox not picking up the conftest.py file in time, I'd like to defer this to a follow up PR if possible. I agree that the solution of specifying the packages in setup.cfg and somehow having that information be copied into the installed version would be nice, but I'd like to investigate that in a dedicated PR. I've opened an issue to keep track of this: #439 - so can we ignore it for the purposes of the PR and consider it an ongoing bug?

Otherwise I think I've addressed all the comments so far, but let me know if I missed something!

[pllim]

LGTM! But should wait for @mhvk too.

[mhvk]

I don't think I'll have time for another detailed look today, so would suggest to just merge this - it was already great as it was and clearly is now even better! And we have follow-up issues raised already.

Super-nice work!

[pllim]

Merged. Thank you, all!