Add 'Literal[...]' to typing_extension stubs

This pull request adds 'Literal' to the typing_extension stubs
and serves as the dual of this PR:
python/typing#591

For background context, here's the associated PEP draft:
Michael0x2a/peps#1

third_party/2and3/typing_extensions.pyi

@@ -21,6 +21,7 @@ def runtime(cls: _TC) -> _TC: ...
 Protocol: _SpecialForm = ...
 Final: _SpecialForm = ...
 def final(f: _F) -> _F: ...
+Literal: _SpecialForm = ...
 
 if sys.version_info >= (3, 3):
     from typing import ChainMap as ChainMap