Adjust ast imports to make pytype happy. (#1617)

Turns out that pytype is a bit more finicky about imports, and
differentiates between '*' imports for export, and imports
for use in the local pyi. This adjusts ast.pyi to make pytype
understand it again.

stdlib/2/ast.pyi

@@ -4,6 +4,7 @@ import typing
 from typing import Any, Iterator, Union
 
 from _ast import *
+from _ast import AST, Module
 
 __version__ = ...  # type: str
 PyCF_ONLY_AST = ...  # type: int