Consider __doc__ always Optional. (#641)

python/mypy#2380 showed a discrepancy between object and FunctionType in stdlib2. The first defined __doc__ to be str, the second Optional[str]. As FunctionType depends on object, this is no longer valid.

As suggested by @gvanrossum in python/mypy#2380, all __doc__ should be considered Optional.

(Final verdict was just to remove most __doc__ attributes since it's inherited from object.)

stdlib/2/ast.pyi

@@ -33,12 +33,10 @@ def literal_eval(node_or_string: Union[str, AST]) -> Any: ...
 def walk(node: AST) -> Iterator[AST]: ...
 
 class NodeVisitor():
-    __doc__ = ...  # type: str
     def visit(self, node: AST) -> Any: ...
     def generic_visit(self, node: AST) -> None: ...
 
 class NodeTransformer(NodeVisitor):
-    __doc__ = ...  # type: str
     def generic_visit(self, node: AST) -> None: ...