GitHub Actions: Add Python 3.14t to the testing

[cclauss]

Python 3.14 (the π version) was released today.

• https://www.python.org/downloads/release/python-3140/
• https://github.com/actions/python-versions/releases

Installing numpy v1 (1.26.4) fails on Python 3.14t. -- https://pypi.org/project/numpy
% uvx --python=3.14 numpy=="1.*" --version # Fails.
--> Upgrade matpotlib

Free-threaded Python 3.14t is blocked by:

• Modernize ci python-hyper/brotlicffi#205
• Support Py_GIL_DISABLED python-hyper/brotlicffi#206 REVERTED
• A release of https://pypi.org/project/brotlicffi > 1.1.0.0 1.2.0.0

Summary

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.