Migrate to pyproject.toml and support Python 3.12.

Bring the project in-line with modern packaging practices. Remove references to the unsupported nose package and replace them with unittest.

reliability enhancements, and support for new message formats

Several plugins have been updated to have more robust message handling. Support has been added for Clair v4 and messagebusd messages. Code is now checked by CodeQL and tested under Python 3.8 and 3.9.

remove python2 support

As of Fedora 31, fedmsg no longer supports python2 or builds rpms for it. This release also drops python2 support, since it is no longer relevant.