Core: test all libraries on Python 3.8

[tswast]

Python 3.8 was released in October. We should test our libraries on 3.8 and once we're comfortable with how they're running mark 3.8 as compatible in the setup.py files.

[tswast]

I know this was blocked on protobuf, but we might be able to start testing with the latest release candidate: https://pypi.org/project/protobuf/3.11.0rc1/