Add stub for stat.filemode (#1837)

Fixes #1836.

stdlib/3/stat.pyi

@@ -16,6 +16,9 @@ def S_ISSOCK(mode: int) -> bool: ...
 def S_IMODE(mode: int) -> int: ...
 def S_IFMT(mode: int) -> int: ...
 
+if sys.version_info >= (3, 3):
+    def filemode(mode: int) -> str: ...
+
 ST_MODE = 0
 ST_INO = 0
 ST_DEV = 0