Declare Python 3.9 support

[hugovk]

Python 3.9.0 is due out on Monday (https://www.python.org/dev/peps/pep-0596/).

We're already testing against 3.9-dev, so let's also declare support for Python 3.9.

Add the 3.9 Trove classifier to setup.py, and also py39 to tox.ini.

---

If doing this after the release:

• Check if 3.9-dev can be replaced with plain 3.9 in .travis.yml

• Also see if the 3.9-dev line can be removed from .github/workflows/test.yml and replaced with "3.9" with the other non-devs.

Important: If doing these, please enable the CIs on your fork and make sure they pass before submitting the PR.

• Travis CI
• GitHub Actions
• Trove classifier
• Tox

[jaimcamp]

Hi, could I take this one?

[hugovk]

@jaimcamp Certainly!

[hugovk]

@jaimcamp How did you get on?

[jaimcamp]

Hi @hugovk , I have been waiting Travis support for 3.9 for a while now.
It's still not there (https://docs.travis-ci.com/user/languages/python/#python-versions), but I have opened a WIP PR, which can be merged as soon as Travis adds support for 3.9 #171

[hugovk]

Travis CI now has 3.9 available.

[jaimcamp]

Added it again in #176 .