Update input parameter type for os.fsencode (+bytes) and os.fsdecode (+str).

Author: sproshev

stdlib/3/os/__init__.pyi

@@ -183,14 +183,14 @@ class statvfs_result:  # Unix only
 
 # ----- os function stubs -----
 if sys.version_info >= (3, 6):
-    def fsencode(filename: Union[str, PathLike]) -> bytes: ...
+    def fsencode(filename: Union[str, bytes, PathLike]) -> bytes: ...
 else:
-    def fsencode(filename: str) -> bytes: ...
+    def fsencode(filename: Union[str, bytes]) -> bytes: ...
 
 if sys.version_info >= (3, 6):
-    def fsdecode(filename: Union[bytes, PathLike]) -> str: ...
+    def fsdecode(filename: Union[str, bytes, PathLike]) -> str: ...
 else:
-    def fsdecode(filename: bytes) -> str: ...
+    def fsdecode(filename: Union[str, bytes]) -> str: ...
 
 if sys.version_info >= (3, 6):
     @overload