[pkgutil] returns a List[str] usually

Yes, technically it returns whatever its first argument is if it isn't
a list.

Doing this because python/mypy#9454 (comment)

Author: hauntsaninja

stdlib/pkgutil.pyi

@@ -18,7 +18,7 @@ if sys.version_info >= (3, 6):
 else:
     _ModuleInfoLike = Tuple[Union[MetaPathFinder, PathEntryFinder], str, bool]
 
-def extend_path(path: Iterable[str], name: str) -> Iterable[str]: ...
+def extend_path(path: List[str], name: str) -> List[str]: ...
 
 class ImpImporter:
     def __init__(self, dirname: Optional[str] = ...) -> None: ...