bpo-33967: Remove use of deprecated assertRaisesRegexp() (GH-8261)

It was added in test_functools at 445f1b3.

Lib/test/test_functools.py

@@ -2310,7 +2310,7 @@ class TestSingleDispatch(unittest.TestCase):
         def f(*args):
             pass
         msg = 'f requires at least 1 positional argument'
-        with self.assertRaisesRegexp(TypeError, msg):
+        with self.assertRaises(TypeError, msg=msg):
             f()
 
 if __name__ == '__main__':