Drop support for Python 3.11 for releases after October 2025

[namurphy]

SPEC 0 recommends that packages in the scientific pythoniverse support minor releases of Python that were made in the last 36 months. Python 3.11 was released in October 2022, so we will be able to drop Python 3.11 support for releases after October 2025.

Tasks

• pyproject.toml
• noxfile.py: versions of Python used for testing
• Search for 3.11 in the docs (and perhaps 3.10 & 3.9 also, just in case)
• Add a changelog entry