Bump macOS build to Python 3.8 and update python 3 versions.

[clokep]

• Changes the macOS Python 3 build to use 3.8 (which is what is in homebrew now).
• Drops support for Python 3.3 and 3.4 as they're EOL.
• Adds support for Python 3.7 and 3.8.

I should note that this is all CI / tox changes. Nothing needs to be changed in the code.

Fixes #20

[clokep]

FTR I also plan to remove Python 2 support (in a separate PR).