smartset: set attribute using sysstr instead of bytes

We do not needs bytes here.

mercurial/smartset.py

@@ -359,10 +359,10 b' class baseset(abstractsmartset):'
         return s
 
     def __and__(self, other):
-        return self._fastsetop(other, b'__and__')
+        return self._fastsetop(other, '__and__')
 
     def __sub__(self, other):
-        return self._fastsetop(other, b'__sub__')
+        return self._fastsetop(other, '__sub__')
 
     def _slice(self, start, stop):
         # creating new list should be generally cheaper than iterating items