Add python 3.9 to CI.

[tirkarthi]

Add Python 3.9 to CI and tox

• Fixes Add python 3.9 support #1684

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[davidism]

This made Windows and Mac builds frustratingly slow. I'm going to revert this until a real 3.9 env is supported.

[askpatrickw]

@davidism This now merged in Github Actions. Can it now be re-commited to Click so downstream dependencies can pick it up?