Support the free-threaded build of Python 3.13/3.14

[ngoldbaum]

See https://py-free-threading.github.io for in-depth documentation on supporting the free-threaded build in a Python package like srsly.

I'm able to build srsly on 3.13.4t and one test fails:

FAILED srsly/srsly/tests/cloudpickle/cloudpickle_test.py::test_extract_class_dict - AssertionError: assert ['C_CONSTANT'...', 'method_c'] == ['C_CONSTANT'...', 'method_c']

If I force the GIL to be disabled and run each test many times in a thread pool using pytest-run-parallel, I see some additional test failures, but most look like they're due to use of global state in the tests?

PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_file_test.py::CloudPickleFileTests::test_plus_mode - FileNotFoundError: [Errno 2] No such file or directory: '/var/folders/nk/yds4mlh97kg9qdq745g715rw0000gn/T/tmper94ulhx/testfile'
PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_file_test.py::CloudPickleFileTests::test_r_mode - FileNotFoundError: [Errno 2] No such file or directory: '/var/folders/nk/yds4mlh97kg9qdq745g715rw0000gn/T/tmp0kdo5j77/testfile'
PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_file_test.py::CloudPickleFileTests::test_seek - FileNotFoundError: [Errno 2] No such file or directory: '/var/folders/nk/yds4mlh97kg9qdq745g715rw0000gn/T/tmpc3amolmn/testfile'
PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_file_test.py::CloudPickleFileTests::test_w_mode - FileNotFoundError: [Errno 2] No such file or directory: '/var/folders/nk/yds4mlh97kg9qdq745g715rw0000gn/T/tmp3lyp2iz3/testfile'
PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_test.py::test_extract_class_dict - AssertionError: assert ['C_CONSTANT'...', 'method_c'] == ['C_CONSTANT'...', 'method_c']
PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_test.py::CloudPickleTest::test_dynamic_module - ModuleNotFoundError: No module named 'mod'
PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_test.py::CloudPickleTest::test_module_locals_behavior - FileNotFoundError: [Errno 2] No such file or directory: '/var/folders/nk/yds4mlh97kg9qdq745g715rw0000gn/T/tmp_cloudpickle_test_p4zmg40...
PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_test.py::CloudPickleTest::test_non_module_object_passing_whichmodule_test - KeyError: 'NonModuleObject'
PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_test.py::CloudPickleTest::test_submodule - NameError: name 'xml' is not defined. Did you forget to import 'xml'?
PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_test.py::CloudPickleTest::test_submodule_closure - KeyError: 'xml'
PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_test.py::Protocol2CloudPickleTest::test_module_locals_behavior - FileNotFoundError: [Errno 2] No such file or directory: '/var/folders/nk/yds4mlh97kg9qdq745g715rw0000gn/T/tmp_cloudpickle_test_he3uh0r...
PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_test.py::Protocol2CloudPickleTest::test_non_module_object_passing_whichmodule_test - KeyError: 'NonModuleObject'
PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_test.py::Protocol2CloudPickleTest::test_submodule - NameError: name 'xml' is not defined. Did you forget to import 'xml'?
PARALLEL FAILED srsly/srsly/tests/cloudpickle/cloudpickle_test.py::Protocol2CloudPickleTest::test_submodule_closure - KeyError: 'xml'
PARALLEL FAILED srsly/srsly/tests/test_msgpack_api.py::test_msgpack_custom_encoder_decoder - TypeError: can not serialize 'CustomObject' object