CI: add python 3.8 testing

[klauer]

Testing on Python 2.7 is failing as the codebase uses a considerable amount of f-strings. Removed that and added Python 3.8.

[prjemian]

I'm OK for adding python 3.8 testing but despite the py27 EOL, we should maintain ability to use that. The f strings can be converted. Any other incompatibilities with py27?

[klauer]

Sad to hear you still want to support 2.7.

There may be other things required to support it, but I didn't look into them. I'll push another commit re-adding it to CI in a bit.

[prjemian]

Thanks.