gh-127020: Make PyCode_GetCode thread-safe for free threading (#127043)

Some fields in PyCodeObject are lazily initialized. Use atomics and
critical sections to make their initializations and accesses thread-safe.

Lib/test/test_free_threading/test_code.py

@@ -0,0 +1,30 @@
+import unittest
+
+from threading import Thread
+from unittest import TestCase
+
+from test.support import threading_helper
+
+@threading_helper.requires_working_threading()
+class TestCode(TestCase):
+    def test_code_attrs(self):
+        """Test concurrent accesses to lazily initialized code attributes"""
+        code_objects = []
+        for _ in range(1000):
+            code_objects.append(compile("a + b", "<string>", "eval"))
+
+        def run_in_thread():
+            for code in code_objects:
+                self.assertIsInstance(code.co_code, bytes)
+                self.assertIsInstance(code.co_freevars, tuple)
+                self.assertIsInstance(code.co_varnames, tuple)
+
+        threads = [Thread(target=run_in_thread) for _ in range(2)]
+        for thread in threads:
+            thread.start()
+        for thread in threads:
+            thread.join()
+
+
+if __name__ == "__main__":
+    unittest.main()