Move typing_extensions to stdlib

I'm not sure what the best way to do this is. This effectively orphans
types-typing-extensions, but maybe that's not really a problem. (We can
yank the package too)

See #5229 (comment)
for some details

Author: hauntsaninja

stdlib/VERSIONS

@@ -233,6 +233,7 @@ tty: 2.7
 turtle: 2.7
 types: 3.6
 typing: 3.6
+typing_extensions: 2.7
 unicodedata: 2.7
 unittest: 3.6
 urllib: 3.6