Add Python 3.11 to CI test matrix

[josh-hadley]

Description

Add Python 3.11 to the testing matrix and update the README.md description of supported Python versions.

Checklist:

• [x] I have followed the Contribution Guidelines
• [x] I have added test code and data to prove that my code functions correctly
• [ ] I have verified that new and existing tests pass locally with my changes
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation