mdiff: remove use of __slots__

The use of __slots__ was added way back in 2006 in 4ec58b157265.
__slots__ isn't necessary for this class.

mercurial/mdiff.py

@@ -58,10 +58,8 b' class diffopts(object):'
         'upgrade': False,
         }
 
-    __slots__ = defaults.keys()
-
     def __init__(self, **opts):
-        for k in self.__slots__:
+        for k in self.defaults.keys():
             v = opts.get(k)
             if v is None:
                 v = self.defaults[k]