Support freethreading Python

[hroncok]

• Check the code for problematic places (how?)
  • On Python 3.13.0a6+, use the %T format in exception messages #17
• Add Py_MOD_GIL_NOT_USED
• Add freethreading jobs to CI (this requires tox 4.26+ available in Fedora 42+ and ideally adding freethreading Pythons to fedora-python-tox)
  • Update to Fedora 42, add freethreading Python fedora-python-tox#87
• Use pytest-run-parallel or alternative (passes for me locally)

[hroncok]

I talked about this with @encukou. Assuming the crypt_r call itself is thread safe¹, the only potentially problematic part of the code is Py_TYPE(args[1])->tp_name:

crypt_r/src/_crypt_r.c

Lines 37 to 40 in a52c167

	PyErr_Format(
	PyExc_TypeError, "crypt argument 1 (word) must be string, not %s",
	Py_TYPE(args[0])->tp_name
	);

1. an argument is passed of type X
2. we call Py_TYPE on it, and get X which does not increment the refcount
3. the type of the argument is changed by another thread (e.g. by assigning to __class__)
4. all other instances of this type are gone, garbage collected, the type is deallocated
5. we call ->tp_name on garbage

This is technically almost impossible to happen but it might :D

A solution is to use the %T format here instead, which likely has the same problem, but the implementation is in Python and is no longer "our problem". %T was added in Python 3.13.

Footnotes

1. Where the r stands for re-entrant, so we assume it is. ↩