modbuiltins: Add NotImplemented builtin constant.
From https://docs.python.org/3/library/constants.html#NotImplemented : "Special value which should be returned by the binary special methods (e.g. __eq__(), __lt__(), __add__(), __rsub__(), etc.) to indicate that the operation is not implemented with respect to the other type; may be returned by the in-place binary special methods (e.g. __imul__(), __iand__(), etc.) for the same purpose. Its truth value is true." Some people however appear to abuse it to mean "no value" when None is a legitimate value (don't do that).