[compat] Mute some F821 under python 3

youtube_dl/compat.py

@@ -2909,8 +2909,8 @@ else:
     if platform.python_implementation() == 'IronPython' and sys.version_info < (2, 7, 8):
         class compat_Struct(struct.Struct):
             def unpack(self, string):
-                if not isinstance(string, buffer):
-                    string = buffer(string)
+                if not isinstance(string, buffer):  # noqa: F821
+                    string = buffer(string)  # noqa: F821
                 return super(compat_Struct, self).unpack(string)
     else:
         compat_Struct = struct.Struct