CPython: Initialize all slots of PyMethodDef table

Created originally on Bitbucket by dalcinl (Lisandro Dalcin)

Was already merged in Bitbucket before import, marked as merged by the import user

Source changeset not longer availableat the time of import. Commit listwill look empty

Initializing PyMethodDef.ml_doc to NULL silence warnings when GCC is invoked with the flag -Wmissing-field-initializers.