Support free threaded Python versions like '3.13t' (#973)

* Support free threaded Python versions like '3.13t'

Python wheels, pyenv, and a number of other tools use 't' in the Python
version number to identify free threaded builds. For example, '3.13t',
'3.14.0a1', '3.14t-dev'.

This PR supports that syntax in `actions/setup-python`, strips the "t",
and adds "-freethreading" to the architecture to select the correct
Python version.

See #771

* Add free threading to advanced usage documentation

* Fix desugaring of `3.13.1t` and add test case.

* Add freethreaded input and fix handling of prerelease versions

* Fix lint

* Add 't' suffix to python-version output

* Use distinct cache key for free threaded Python

* Remove support for syntax like '3.14.0a1'

* Clarify use of 't' suffix

* Improve error message when trying to use free threaded Python versions before 3.13