Update Python versions in Appveyor and Travis CI configurations

[mdickinson]

We need to update the Python versions in our continuous integration configurations:

For Appveyor: add Python 3.9 when available
For Travis CI: change 3.9-dev to 3.9 when Python 3.9 is available

See also #1303, which updates the Python versions used for GitHub Actions.

[mdickinson]

Related upstream issue about supporting Python 3.9 on Appveyor: appveyor/ci#3541

I was unable to find anything equivalent for Travis CI.

[mdickinson]

I edited the description to only mention the 3.9 changes; #1303 should fix the rest (dropping Python 3.5, adding Python 3.8 for Appveyor).

[hugovk]

This can be closed now?

[mdickinson]

Yes it can; thanks for the reminder!

Closing because we no longer make use of Travis CI or Appveyor; all continuous integration is run on GitHub Actions, and tests are run on versions up to and including 3.10-dev.