smartset: use native string when peeking in __dict__

# skip-blame just an r prefix on a string literal

Differential Revision: https://phab.mercurial-scm.org/D1897

mercurial/smartset.py

@@ -306,7 +306,7 b' class baseset(abstractsmartset):'
         self._istopo = False
 
     def __len__(self):
-        if '_list' in self.__dict__:
+        if r'_list' in self.__dict__:
             return len(self._list)
         else:
             return len(self._set)