tests: auto skip cffi backend on 3.13 free-threaded builds

cffi doesn't support free-threading until the 2.0 release. And the
2.0 release only implements it on 3.14+. So refuse to run tests against
the cffi backend on 3.13 free-threaded builds.

Author: indygreg

tests/conftest.py

@@ -1,11 +1,26 @@
 import os
+import sys
+import sysconfig
+
+import pytest
 
 try:
     import hypothesis  # type: ignore
 except ImportError:
     hypothesis = None  # type: ignore
 
 
+def pytest_configure(config):
+    if (
+        os.environ.get("PYTHON_ZSTANDARD_IMPORT_POLICY", "").strip() == "cffi"
+        and sysconfig.get_config_var("Py_GIL_DISABLED") == 1
+        and sys.version_info[0:2] == (3, 13)
+    ):
+        pytest.exit(
+            "cffi backend not supported on 3.13 free-threaded Python", 0
+        )
+
+
 if hypothesis:
     default_settings = hypothesis.settings(deadline=10000)
     hypothesis.settings.register_profile("default", default_settings)