This patch still(!) applies almost perfectly cleanly to trunk.  On a 64-
bit machine, I'm getting a failure in test_auto_overflow, coming from:

>>> pow(0L, 0, 9223372036854775807)
28051505152L

I haven't looked hard to figure out where this is coming from, but my 
guess is that the 15-bitness of digits is hard-coded in the patch 
somewhere.

My general feeling is that three-argument pow is such a little-used 
operation in Python that it's not worth the extra code to speed it up.