fix special methods testcase for previous changes to __ipow__

tests/run/special_methods_T561.pyx

@@ -401,7 +401,7 @@ cdef class VerySpecial:
     def __imod__(self, other):
         print "VS __imod__ %d %%= %d" % (self.value, other.value)
 
-    def __ipow__(self, other, mod):
+    def __ipow__(self, other):
         # We must declare mod as an argument, but we must not touch it
         # or we'll get a segfault.  See #562
         print "VS __ipow__ %d %d" % (self.value, other.value)