Add support for Python 3.9

[hugovk]

Tracking issue for getting the pieces into place to support Python 3.9 in Pillow 8.0.0, scheduled for 2020-10-15.

• Wait for 3.9.0: https://discuss.python.org/t/python-3-9-0-is-now-available-and-you-can-already-test-3-10-0a1/5375?u=hugovk
• Wait for 3.9 on GHA: Add Python 3.9 support actions/setup-python#148 (comment)
• Wait for 3.9 on Travis CI: https://travis-ci.community/t/python-3-9-0-build/10091?u=hugovk
• Wait for 3.9 on AppVeyor: Python 3.9 appveyor/ci#3541
• 3.9: Test GitHub Actions #4951 Replace 3.9-dev with 3.9 on GHA
• Travis CI: Test on Python 3.9 final #5010 Replace 3.9-dev with 3.9 on Travis CI
• AppVeyor: Test on Python 3.9 final #5014 Replace 3.8 with 3.9 on AppVeyor
• 3.9: Add Trove classifier, update setup.py version check, add to tox.ini #4952 Add Trove classifier, update setup.py version check, add to tox.ini
• Update support table https://pillow.readthedocs.io/en/latest/installation.html#platform-support
• Updated CI targets #4961 Update CI targets https://pillow.readthedocs.io/en/latest/installation.html#continuous-integration-targets
• Added Python 3.9 pillow-wheels#164 Update https://github.com/python-pillow/pillow-wheels

[evandrix]

Pillow-7.2.0-cp39-cp39-win_amd64.whl.zip

[cclauss]

3.9-dev on Travis CI is currently installing Python 3.9.0+

[hugovk]

Yep, on Travis -dev is a daily build from the corresponding branch.

(On GHA, -dev is the last tagged release.)

Looks like we have the minimum in place for the release.