Merge pull request #322 from amitsaha/stringio_len

stdlib: StringIO - Add len attribute issue #316

stdlib/2.7/StringIO.pyi

@@ -5,6 +5,7 @@ from typing import Any, IO, AnyStr, Iterator, Iterable, Generic, List
 class StringIO(IO[AnyStr], Generic[AnyStr]):
     closed = ... # type: bool
     softspace = ... # type: int
+    len = ... # type: int
     def __init__(self, buf: AnyStr = ...) -> None: ...
     def __iter__(self) -> Iterator[AnyStr]: ...
     def next(self) -> AnyStr: ...