Make sys.getfilesystemencoding not return None (#1245)

Fixes #1221

Author: euresti

stdlib/2/sys.pyi

@@ -124,7 +124,7 @@ def exit(arg: Any = ...) -> NoReturn:
 def getcheckinterval() -> int: ...  # deprecated
 def getdefaultencoding() -> str: ...
 def getdlopenflags() -> int: ...
-def getfilesystemencoding() -> Union[str, None]: ...
+def getfilesystemencoding() -> str: ...  # In practice, never returns None
 def getrefcount(arg: Any) -> int: ...
 def getrecursionlimit() -> int: ...
 def getsizeof(obj: object, default: int = ...) -> int: ...