[3.15] Use _testlimitedcapi in test_free_threading.test_capi (#158935) (#159146)

Use _testlimitedcapi in test_free_threading.test_capi (#158935)

Replace ctypes with _testlimitedcapi. ctypes is not always available,
and _testlimitedcapi is a thin wrapper to the C API.

Skip the test if _testlimitedcapi is missing.

(cherry picked from commit 1818fba)

Author: vstinner

Lib/test/test_free_threading/test_capi.py

@@ -1,14 +1,12 @@
-import ctypes
 import sys
 import unittest
 
+from test.support import import_helper
 from test.support import threading_helper
 from test.support.threading_helper import run_concurrently
 
-
-_PyImport_AddModuleRef = ctypes.pythonapi.PyImport_AddModuleRef
-_PyImport_AddModuleRef.argtypes = (ctypes.c_char_p,)
-_PyImport_AddModuleRef.restype = ctypes.py_object
+# Skip the test if the extension is missing
+_testlimitedcapi = import_helper.import_module('_testlimitedcapi')
 
 
 @threading_helper.requires_working_threading()
@@ -20,13 +18,14 @@ def test_pyimport_addmoduleref_thread_safe(self):
         NUM_ITERS = 10
         NTHREADS = 4
 
+        PyImport_AddModuleRef = _testlimitedcapi.PyImport_AddModuleRef
         module_name = f"test_free_threading_addmoduleref_{id(self)}"
         module_name_bytes = module_name.encode()
         sys.modules.pop(module_name, None)
         results = []
 
         def worker():
-            module = _PyImport_AddModuleRef(module_name_bytes)
+            module = PyImport_AddModuleRef(module_name_bytes)
             results.append(module)
 
         for _ in range(NUM_ITERS):