Build with Python 3.11 on CI and RTD

[hugovk]

Build the docs on GitHub Actions and Read the Docs using the new Python 3.11.

[AA-Turner]

An alternate is to use "3" for both to use the latest stable Python, but this looks good for explicitness and simplicity.

A

[ezio-melotti]

I think 3, or even 3-dev (if it's going to automatically pick 3.12a/b/rc once they are available) would be better.

[hugovk]

There is no 3-dev, so 3 it is.

Demo: https://github.com/hugovk/test/actions/runs/3464785635

[hugovk]

RTD also supports 3 for the latest version, so updated that to match:

• https://docs.readthedocs.io/en/stable/config-file/v2.html#build-tools-python
• https://readthedocs.org/projects/cpython-devguide/builds/18635085/