This patch adds an optional default value to dict.pop,
so that it parallels dict.get, see discussion in bug
639806.

If no default is given, the old behavior still exists,
so backwards compatibility is no problem.
The new pop must use METH_VARARGS
and PyArg_UnpackTuple, somewhat effecting
efficiency.

If this is considered desirable, I could also
provide the same behavior for list.pop.