Add Python 3.8 support

[hugovk]

Python 3.8.0 was released yesterday, 2019-10-14:

• https://discuss.python.org/t/python-3-8-0-is-now-available/2478

Please could you also add support in GitHub Actions for Python 3.8? Thank you!

Also requested at GitHub Community Forum:

• https://github.bokerqi.topmunity/t5/GitHub-Actions/Add-Python-3-8-support/td-p/34759

Thank you!

[webknjaz]

@damccorm ^

[damccorm]

@bryanmacfarlane who owns this now

[matthewfeickert]

cc @kratsg @lukasheinrich