[MNT] Validate Python 3.14t (free threading) on CI/GHA

[amotl]

Hi. Python provides GA releases of its free-threading variant starting with Python 3.13. This patch makes CI/GHA validate the codebase against Python3.14t, which, I believe, will be good enough.

Starting with the 3.13 release, CPython has support for a build of
Python called free threading where the global interpreter lock (GIL)
is disabled.

-- https://docs.python.org/3/howto/free-threading-python.html

[amotl]

Currently, this does not work on Windows due to a dependency problem.

╰─▶ Because pywin32{sys_platform == 'win32'}>=304 has no wheels with
a free-threading compatible ABI tag and docker>=6.0.0b1 depends
on pywin32{sys_platform == 'win32'}>=304, we can conclude that
docker>=6.0.0b1 cannot be used. (1)

The CI build also fails on Ubuntu:

× Failed to download and build pmdarima==2.1.1
╰─▶ Package metadata version 0.0.0 does not match given version 2.1.1