fix: Python 3.13t with GIL (#5139)

* ci: try Python 3.13t

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* fix: support Python 3.13t

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* fix: patch PyPy

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* tests: one more int cast

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* tests: cleanup

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* refactor: use named constant in tests for immortal refcounts

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* docs: move comment about free threaded Python

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

---------

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>