Skip abi3 tests when freethreading Python build is used

Skip the tests using abi3 extension when the freethreading build is
used, as Python 3.13 freethreading does not support the limited API
and its use results in a build error.

Fixes #636

Author: mgorny

tests/conftest.py

@@ -7,6 +7,7 @@
 import os.path
 import subprocess
 import sys
+import sysconfig
 
 import pytest
 
@@ -22,7 +23,7 @@ def wheels_and_eggs(tmp_path_factory):
         "unicode.dist",
     )
 
-    if sys.platform != "win32":
+    if sys.platform != "win32" and sysconfig.get_config_var("Py_GIL_DISABLED") != 1:
         # ABI3 extensions don't really work on Windows
         test_distributions += ("abi3extension.dist",)