Fix the signature of IOBase._checkClosed(). (#3305)

* In Python 2, the optional `msg` argument was missing.
* In Python 3, the method was missing altogether.

Reference:
https://github.com/python/cpython/blob/2.7/Lib/_pyio.py#L423
https://github.com/python/cpython/blob/3.6/Lib/_pyio.py#L443

stdlib/3/io.pyi

@@ -45,6 +45,7 @@ class IOBase:
     def __del__(self) -> None: ...
     @property
     def closed(self) -> bool: ...
+    def _checkClosed(self, msg: Optional[str] = ...) -> None: ...  # undocumented
 
 class RawIOBase(IOBase):
     def readall(self) -> bytes: ...