Nit fixes, cleanup old versions

[jwhitlock]

This fixes a few nits and warnings I noticed when running locally:

• I'm ignoring dump.rdb, created when I run redis-server in the directory so tests can access it.
• The documentation build complained about language = None. The default of en works for us. Also bumped the copyright.
• Added Framework :: Django :: 4.2 to the classifiers
• Django 3.2 doesn't claim support for Python 3.11. Tests pass, but there are some deprecation warnings. I removed it from the matrix.
• I removed the dropped Flask and Sanic versions from the commands
• I moved the Django-specific pytest options from pytest.ini to tox.ini, to remove some warnings from the Flask and Sanic builds about unknown options.

There's more I want to do (convert from setup.py to pyproject.toml, figure out why a tox run opens so many files), but I'm stopping myself.

All these are nits, so I'm willing to drop any controversial ones.

[jwhitlock]

I've answered some questions, fixed some issues, and also added a .readthedocs.yaml to prepare for this file being required in September: https://blog.readthedocs.com/migrate-configuration-v2/#migrate-your-project-to-readthedocs-yaml-configuration-file-v2

Update - since the new default Python in RTD is Python 3.11, I also updated the doc build to use 3.11.