Add free-threaded CPython (3.14t) support

[kumaraditya303]

Hello, I am a CPython Core Developer working on free-threading support. This PR adds support for free-threading on 3.14t+ (#749).

Changes:

• Mark all Cython extensions with freethreading_compatible=True so the interpreter no longer re-enables the GIL when numcodecs is imported
• Require Cython>=3.1, needed for the freethreading_compatible directive
• blosc: replace the lazily created multiprocessing.Lock (and get_mutex()) with a module-level threading.Lock, reset in forked children via os.register_at_fork
• blosc: take the lock in _init, _destroy, set_nthreads and the global context path of decompress, so blosc_set_nthreads cannot re-create the global context while another thread is using it
• blosc: compute the compressor list once at import; blosc_list_compressors() fills a static buffer without locking and was called on every compress()
• CI: add 3.14t to the test matrix, installing without pcodec, zfpy and google_crc32c which do not ship free-threaded wheels yet
• Wheels: enable cpython-freethreading in cibuildwheel, build cp314t-* wheels, and assert in the wheel test that the GIL is not re-enabled on free-threaded builds

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (82c3635) to head (0021eae).

Additional details and impacted files

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##              main      #858   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           27        27           
  Lines          905       905           
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  Hits           905       905           

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[kumaraditya303]

I dropped -l from shell so that setup-python's PATH order is respected and free-threaded Python is installed correctly, otherwise it incorrectly picks up gil enabled python on macos 3.14t.