ast.get_docstring: fix return type (#4595)

Resolves #4575

Author: mitch292

stdlib/3/ast.pyi

@@ -191,7 +191,7 @@ else:
     def dump(node: AST, annotate_fields: bool = ..., include_attributes: bool = ...) -> str: ...
 
 def fix_missing_locations(node: _T) -> _T: ...
-def get_docstring(node: AST, clean: bool = ...) -> str: ...
+def get_docstring(node: AST, clean: bool = ...) -> Optional[str]: ...
 def increment_lineno(node: _T, n: int = ...) -> _T: ...
 def iter_child_nodes(node: AST) -> Iterator[AST]: ...
 def iter_fields(node: AST) -> Iterator[_typing.Tuple[str, Any]]: ...

third_party/3/typed_ast/ast3.pyi

@@ -12,7 +12,7 @@ def parse(source: Union[str, bytes], filename: Union[str, bytes] = ..., mode: st
 def copy_location(new_node: AST, old_node: AST) -> AST: ...
 def dump(node: AST, annotate_fields: bool = ..., include_attributes: bool = ...) -> str: ...
 def fix_missing_locations(node: AST) -> AST: ...
-def get_docstring(node: AST, clean: bool = ...) -> str: ...
+def get_docstring(node: AST, clean: bool = ...) -> Optional[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]]: ...