[3.12] gh-76785: Make test.test_interpreters executable (GH-112982) (GH-113470)

This is so that we can run python -m test.test_interpreters. As such it
backports that aspect of commit 86a77f4e1a,
where it is implemented by a package __main__.py.

Lib/test/test_interpreters.py

@@ -1053,3 +1053,7 @@ class TestSendRecv(TestBase):
         self.assertEqual(obj4, b'spam')
         self.assertEqual(obj5, b'eggs')
         self.assertIs(obj6, default)
+
+
+if __name__ == "__main__":
+    unittest.main()