__rop__ now takes precendence over __op__.  Those circumstances are:

  - Both arguments are new-style classes
  - Both arguments are new-style numbers
  - Their implementation slots for tp_op differ
  - Their types differ
  - The right argument's type is a subtype of the left argument's type

Also did this for the ternary operator (pow) -- only the binary case
is dealt with properly though, since __rpow__ is not supported anyway.