• Add CFLAGS used on Travis CI (-Werror, std=c99 and similar) to tox environments
• Add std=c90 environments for Python versions that compile with c90
• Add an environment for pypy3 and for Python 3.10 (a bit early, but unlikely to break now)
• Some general clean-up (remove python2 remnants, fix comments)
• Add tox-on-Fedora runs to the CI

Partially addresses: #418

The actions might not run on the PR that adds them, but they passed here: https://github.com/encukou/python-ldap/actions/runs/983127874