string is special in that it is a c++ type which has tp_as_number and tp_as_sequence.
This causes problems because when we fixup the slot dispatcher we will set the tp_as_number fields but not the
tp_as_sequence because setting both can cause problems.
Some extensions (e.g. numpy) require that we use the sq_* functions instead of nb_*.
Therefore clear the tp_as_number fields (except nb_remainder which cpython has set too because it is not part of
tp_as_sequence).