win32mbcs: fix a `str` type conditional for py3

Differential Revision: https://phab.mercurial-scm.org/D7535

hgext/win32mbcs.py

@@ -77,7 +77,7 b' configitem('
 
 
 def decode(arg):
-    if isinstance(arg, str):
+    if isinstance(arg, bytes):
         uarg = arg.decode(_encoding)
         if arg == uarg.encode(_encoding):
             return uarg