py3: compare against bytes instead of str

Differential Revision: https://phab.mercurial-scm.org/D2152

hgext/mq.py

@@ -650,7 +650,7 b' class queue(object):'
         self.seriesdirty = True
 
     def pushable(self, idx):
-        if isinstance(idx, str):
+        if isinstance(idx, bytes):
             idx = self.series.index(idx)
         patchguards = self.seriesguards[idx]
         if not patchguards: