chore: update codebase for modern python

[miketheman]

We're on Python 3.7 and higher, so we can clean up any artifacts left from previously-supported Python eras.

Most of the heavy lifting was done with pyupgrade followed by unused imports removal (thanks to flake8 check).

Refs: https://pypi.org/project/pyupgrade/

Closes #203

Signed-off-by: Mike Fiedler miketheman@gmail.com

[miketheman]

While I was here, I replaced os.path usages with pathlib (see refactor commit)

[miketheman]

And another tweak to remove a setuptools warning.