Tweak how ast.pyi imports typing (#2668)

When we import typeshed internally at Dropbox, somehow the fact that
these files are all stubs gets lost (it's a long story...).  This
causes errors like this:

  .../stdlib/2/ast.pyi:6: error: Name 'typing' already defined (by an import)

The quickest way around this is to rename the import to _typing.

stdlib/3/ast.pyi

@@ -1,7 +1,10 @@
 # Python 3.5 ast
 
-import typing
-from typing import Any, Union, Iterator, Optional
+# Rename typing to _typing, as not to conflict with typing imported
+# from _ast below when loaded in an unorthodox way by the Dropbox
+# internal Bazel integration.
+import typing as _typing
+from typing import Any, Iterator, Optional, Union
 
 from _ast import *
 
@@ -19,7 +22,7 @@ def fix_missing_locations(node: AST) -> AST: ...
 def get_docstring(node: AST, clean: bool = ...) -> str: ...
 def increment_lineno(node: AST, n: int = ...) -> AST: ...
 def iter_child_nodes(node: AST) -> Iterator[AST]: ...
-def iter_fields(node: AST) -> Iterator[typing.Tuple[str, Any]]: ...
+def iter_fields(node: AST) -> Iterator[_typing.Tuple[str, Any]]: ...
 def literal_eval(node_or_string: Union[str, AST]) -> Any: ...
 def walk(node: AST) -> Iterator[AST]: ...