Add stubs for typing_extensions module (#1471)

third_party/2and3/typing_extensions.pyi

@@ -0,0 +1,26 @@
+import sys
+import typing
+from typing import ClassVar as ClassVar
+from typing import ContextManager as ContextManager
+from typing import Counter as Counter
+from typing import DefaultDict as DefaultDict
+from typing import Deque as Deque
+from typing import NewType as NewType
+from typing import NoReturn as NoReturn
+from typing import overload as overload
+from typing import Text as Text
+from typing import Type as Type
+from typing import TYPE_CHECKING as TYPE_CHECKING
+
+if sys.version_info >= (3, 3):
+    from typing import ChainMap as ChainMap
+
+if sys.version_info >= (3, 5):
+    from typing import AsyncIterable as AsyncIterable
+    from typing import AsyncIterator as AsyncIterator
+    from typing import AsyncContextManager as AsyncContextManager
+    from typing import Awaitable as Awaitable
+    from typing import Coroutine as Coroutine
+
+if sys.version_info >= (3, 6):
+    from typing import AsyncGenerator as AsyncGenerator