add annotations to open()
diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 3fd1afe..0ef6822 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -35,8 +35,9 @@
self.characters_written = characters_written
-def open(file, mode="r", buffering=None, encoding=None, errors=None,
- newline=None, closefd=True):
+def open(file: (str, bytes), mode: str = "r", buffering: int = None,
+ encoding: str = None, errors: str = None,
+ newline: str = None, closefd: bool = True) -> "IOBase":
r"""Open file and return a stream. Raise IOError upon failure.