feat: Do not use C speedups on Python 3.13+ freethreading

Do not enable C speedups by default if running Python 3.13+
in freethreading mode (with GIL disabled).  Since the extension
does not support this mode explicitly at the moment, loading it would
cause CPython to reenable GIL with a warning and penalize the whole
environment.

Discussed in issue #330.

Author: mgorny

CHANGES.rst

@@ -5,6 +5,11 @@
 7.1.1 (unreleased)
 ==================
 
+- Disable automatically using C speedups when Python 3.13 is used
+  in freethreading mode for the time being (at least until the C
+  extension gains support for freethreading mode).
+  (`#330 <https://github.com/zopefoundation/zope.interface/issues/330>`_)
+
 
 7.1.0 (2024-10-10)
 ==================

src/zope/interface/_compat.py

@@ -74,15 +74,19 @@ def _should_attempt_c_optimizations():
     """
     Return a true value if we should attempt to use the C optimizations.
 
-    This takes into account whether we're on PyPy and the value of the
-    ``PURE_PYTHON`` environment variable, as defined in `_use_c_impl`.
+    This takes into account whether we're on PyPy, whether you're using
+    a freethreading version of Python 3.13+ (where importing
+    the speedups would enable GIL) and the value of the ``PURE_PYTHON``
+    environment variable, as defined in `_use_c_impl`.
     """
     is_pypy = hasattr(sys, 'pypy_version_info')
 
     if _c_optimizations_required():
         return True
     if is_pypy:
         return False
+    if sys.version_info >= (3, 13) and not sys._is_gil_enabled():
+        return False
     return not _c_optimizations_ignored()