Small polish and CI

[akx]

This PR:

• wraps Pushover in a package (for future changes)
• adds some missing entries to the repo's .gitignore
• runs 2to3 manually on the repo then removes use_2to3 from setup.py
  • this makes it possible to use pip install -e . (or python setup.py develop); with use_2to3, it will always 2to3 files into a secondary locations
• adds a very simple CI workflow to at least lint the code with flake8