Update noxfile.py to have better support for Windows

[Peilonrayz]

When developing I run nox -r before finishing a commit so I know that I can push without problems. Whilst I find and fix most errors, docs and cover never complete successfully. This is causing fractured PRs, as I don't know coverage has fallen - as it always errors - until it's too late.

Coverage

Coverage fails to run on the following lines.

• nox/virtualenv.py

  • 189 (186->189)
    Changing this to a ternary allows for 100% coverage.
  • 212 (211->212)
  • 352 (351->352)

• nox/workflow.py

  • 57 (44->57)

Docs

1. Change rm -rf to shutil.rmtree.

2. Change the Python version to 3.7 because sphinx-autobuild, which relies on tornado, no longer works on Windows on 3.8.

3. Sphinx fails to follow the reference to changelog and contributing.
   Since the files don't have a heading this causes some warnings. Since the -W flag is passed this elevates them to errors.

   I have found two solutions:

   • Converting everything to rst, and use include.
   • Don't use the -W flag on Windows.

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Most of the fixes are rather straight forward. However Docs (3) seems controversial, how would you like me to approach it?