Add twine check and add README_PYTHON.rst to doc8 linting

[emilyanndavis]

Description

Resolves #1582.

1. Added twine check task, which validates the source distribution and raises an error if there are problems in the long description that will prevent proper rendering on PyPI. (See twine check example output from an intentionally failed run.)
2. Also updated the check-history-rst-syntax so that it also lints README_PYTHON.rst. While twine check seems to bail after the first syntax error it encounters, doc8 reports all issues, which would be helpful in the (albeit unlikely) scenario in which README_PYTHON.rst is updated and ends up with multiple syntax issues. (See doc8 example output from an intentionally failed run.)

I realize there's a lot of overlap between these two tasks, so I'm open to other options if folks think it's not worthwhile to include both.

Checklist

- [ ] Updated HISTORY.rst and link to any relevant issue (if these changes are user-facing)
- [ ] Updated the user's guide (if needed)
- [ ] Tested the Workbench UI (if relevant)

[phargogh]

Makes sense to me! I just had one very minor suggestion about moving the twine requirement to the environment setup step, but please do let me know if this doesn't make sense or is better the way you have it!

[phargogh]

Would it be worth including twine under the requirements list up above, when we're using the actions setup_env step? I don't think it'd be strictly necessary, but I think it might avoid the need to re-resolve the dependencies, which could potentially save some small amount of time. I think the bigger benefit might be just having all of the python dependencies in once place.

So I'm thinking something like this up above:

      - uses: ./.github/actions/setup_env
        with:
          python-version: ${{ matrix.python-version }}
          requirements-files: requirements.txt
          requirements: ${{ env.CONDA_DEFAULT_DEPENDENCIES }} twine

[emilyanndavis]

Great idea, thanks!

[phargogh]

Thanks @emilyanndavis !