* 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>

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