Switch usages of mypy_extensions.NoReturn over to typing.NoReturn. (#1942)

* Change mypy_extensions.NoReturn to typing.NoReturn everywhere.

third_party/2/six/__init__.pyi

@@ -4,12 +4,11 @@ from __future__ import print_function
 
 import types
 from typing import (
-    Any, AnyStr, Callable, Dict, Iterable, Mapping, Optional,
+    Any, AnyStr, Callable, Dict, Iterable, Mapping, NoReturn, Optional,
     Pattern, Tuple, Type, TypeVar, Union, overload, ValuesView, KeysView, ItemsView
 )
 import typing
 import unittest
-from mypy_extensions import NoReturn
 
 # Exports
 from __builtin__ import unichr as unichr