Test setup for Python 3.9

[uranusjr]

Python 3.9.0 has been released. pip should add it into the test matrix.

This is more of a todo to myself, but feel free to work on it before I can!

TODO:

• Update tox setup
• Update nox setup
• Update Azure setup
• Update Travis setup

Optional:

• Add allow-failure Travis setup for Python HEAD?

[srinivasraj12]

New syntax features:

PEP 584, union operators added to dict;

PEP 585, type hinting generics in standard collections;

PEP 614, relaxed grammar restrictions on decorators.

[uranusjr]

Did Hacktoberfest just change its metric from PRs to issue comments? /s

[McSinyx]

No, and I find it a bit unwelcoming to new (potential) contributor for us to make such assumption. Related to @srinivasraj12 comment, I think there should be a way to track stuff we (wanna) use from newer Python and make compat wrapper for, for the ease of later deprecation. Just an idea, I'm too burnt out at the moment to put together any coherent thought.

[pradyunsg]

FWIW, we generally can't really use the features in newer versions of Python -- since we're stuck with using the lowest-common-denominator portions of all our supported Python versions.

[pradyunsg]

CI stuff isn't done yet. We got the rest covered w/ #8971!

[hugovk]

Tracking issues to follow/upvote:

• Azure Pipelines: Ubuntu ready, macOS/Windows due early this week
  Add Python 3.9 actions/runner-images#1740 (comment)
• Travis CI: no word whatsoever on schedule. 3.9-dev is available for daily builds
  https://travis-ci.community/t/python-3-9-0-build/10091?u=hugovk

[ichard26]

Is it OK to close this issue given that GHA is now handling all of CI and seems to be testing 3.9 just fine, if so, let's close this :)