Use of bin_op in rings_pari_ring.py

[fschulze]

While adding doctest for #12550 I found the following.

sage: R = Pari
Pari       PariError  PariGroup  PariRing   
sage: R = PariRing()
sage: a = R(2)
sage: a^2
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)

/home/agag/schulze/<ipython console> in <module>()

/home/schulze/sage-5.1.beta3/local/lib/python2.7/site-packages/sage/rings/pari_ring.pyc in __pow__(self, other)
     69     def __pow__(self, other):
     70         if not isinstance(other, Pari):
---> 71             return bin_op(self, other, operator.pow)
     72         return self.__class__(self.__x ** other.__x, parent=_inst)
     73 

NameError: global name 'bin_op' is not defined
sage: 

This would be easy to fix using using an explicit Pari(other). Would this be the correct fix or should one do something else using the coercen system?