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

* Change mypy_extensions.NoReturn to typing.NoReturn everywhere.

stdlib/2/os/__init__.pyi

@@ -7,10 +7,9 @@ from posix import stat_result as stat_result  # TODO: use this, see https://gith
 import sys
 from typing import (
     Mapping, MutableMapping, Dict, List, Any, Tuple, Iterator, overload, Union, AnyStr,
-    Optional, Generic, Set, Callable, Text, Sequence, IO, NamedTuple, TypeVar
+    Optional, Generic, Set, Callable, Text, Sequence, IO, NamedTuple, NoReturn, TypeVar
 )
 from . import path as path
-from mypy_extensions import NoReturn
 
 _T = TypeVar('_T')