Mark 3.12 support in release

Also upgrade pre-commit configuration and fix
a warning from flake8

Trivial updates for Python 3.11

Minor tweak to setup.py for the project links

Explicitly mark 3.10 as suppported

Update version in setup.cfg

Trivial updates

0.15 isn't released yet, hence no need to update the version number.

Remove use of iteritems in GraphAlgo as this won't work on Python 3

Last minute changes for a 0.13 release

- Bug in test suite
- Use tox for testing

Create 0.12 maintainance branch