Python3

Based on !74 (closed) and https://gitlab.com/globin/postorius/tree/python3

Thanks to Robin Globster and Simon Hanna for most of the changes in this branch.

This branch currently does work on Python 3 and all tests pass. I am working on improving the test suite for now and will be ready once I get to ~85-90% coverage. I am also improving some tests thats to be a bit more extensive in testing successes and failures.

Changelog for this MR:

• Move code from Python 2 to Python 3 (only).
• Split forms.py into smaller modules.
• Split test_forms.py into smaller modules to test better.
• Improve test coverage.