Use released Python 3.9 instead of prerelease

[mdickinson]

Python 3.9 has been released, so we should update the GitHub Actions continuous integration to run on 3.9 proper instead of on a pre-release version.

This PR updates the GitHub Actions test script accordingly.

xref: actions/setup-python#148

We should also update the Travis and Appveyor scripts, but I'm having difficulty finding evidence that the systems support Python 3.9 yet. I'll give it a try once the GitHub Actions change has completed.

---

EDIT: Updated to also drop Python 3.5 support for both Appveyor and Travis CI.

[mdickinson]

For Appveyor, looks like we don't have 3.9 yet: appveyor/ci#3541. I suspect that's also true for Travis. I'll open an issue to remind us to update those eventually.

[mdickinson]

Updated to drop Python 3.5 runs and add Python 3.8 runs for Appveyor, partially addressing #1304.

[mdickinson]

There still seem to be issues with PySide 2 on Windows (both 32-bit and 64-bit). Here's the error for the 64-bit run (the error for the 32-bit run is almost identical).

PySide2/__init__.py: Unable to import shiboken2 from C:\projects\traits\testdir, C:\Python38-x64\python38.zip, C:\Python38-x64\DLLs, C:\Python38-x64\lib, C:\Python38-x64, C:\Python38-x64\lib\site-packages

[rahulporuri]

LGTM

[rahulporuri]

Can you please update the description to mention that 3.5 was dropped from CI before merging?

[mdickinson]

Can you please update the description to mention that 3.5 was dropped from CI before merging?

Done.

Before merging, I'm just going to quickly try replacing 3.9-dev with 3.9 for Travis. It probably won't work, but I can't find any information online about 3.9 support for Travis CI, and this seems like the quickest way to find out whether it's supported.

[mdickinson]

Failed as expected:

Downloading archive: https://storage.googleapis.com/travis-ci-language-archives/python/binaries/ubuntu/16.04/x86_64/python-3.9.tar.bz2
0.09s$ curl -sSf --retry 5 -o python-3.9.tar.bz2 ${archive_url}
curl: (22) The requested URL returned error: 404 Not Found
Unable to download 3.9 archive. The archive may not exist. Please consider a different version.

[mdickinson]

Random(?) failure on the Python 3.6 Linux Travis CI job:

Warning, treated as error:
failed to reach any of the inventories with the following issues:
intersphinx inventory 'https://docs.enthought.com/pyface/objects.inv' not fetchable due to <class 'requests.exceptions.ConnectionError'>: ('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer'))
The command "pushd docs && python3 -m sphinx -b html -W source build && popd" exited with 2.

I'll restart that job.