Guard call to tp_iternext slot function with CYTHON_USE_TYPE_SLOTS
Guard call to tp_iternext slot function with CYTHON_USE_TYPE_SLOTS

Fixes a segfault on pypy3 due to the coroutine type missing tp_iternext in 5.8.

cython/cython,da-woods/cython,cython/cython,scoder/cython,cython/cython,cython/cython,scoder/cython,da-woods/cython,scoder/cython,da-woods/cython,scoder/cython,da-woods/cython