The leak can reproduce only using this test:

    def test_execve_invalid_env(self):
        args = [sys.executable, '-c', 'pass']

        # equal character in the enviroment variable name
        newenv = os.environ.copy()
        newenv["FRUIT=ORANGE"] = "lemon"
        with self.assertRaises(ValueError):
            os.execve(args[0], args, newenv)

It seems like the bug is in parse_envlist() function of Modules/posixmodule.c, when a key contains the '=' character.