Unlike every other slot, power slots are ternary. Lots of special casing
had to be done in generate_bin_op_wrapper() to support the third slot
argument.

- Ternary pow() does NOT fallback to __rpow__ if __pow__ returns
  NotImplemented unlike binary ops.
- Ternary pow() does NOT try the right operand's __rpow__ first if it's
  a subclass of the left operand and redefines __rpow__ unlike binary
  ops.

Add in the fact it's allowed and common to only define __(r|i)pow__ to
take two arguments and you get a mess of a patch...