Issue 4910, patch 2 of (probably) 3: pave the way for renaming of

Issue 4910, patch 2 of (probably) 3: pave the way for renaming of
nb_long:  remove last remaining use of nb_long
(in the struct module) from the core, set nb_long slots on all builtin
and extension types to 0, and remove uses of __long__ in test_complex
and test_binop.

Reviewed by Benjamin Peterson.