slp_prepare_slots() called twice

The function slp_prepare_slots(PyTypeObject * type) gets called from two places.

1. From init_stackless_methods(). Here init_stackless_methods() sets Py_TPFLAGS_HAVE_STACKLESS_EXTENSION before calling slp_prepare_slots(). Everything is fine.

2. From inherit_slots() in typeobject.c by the COPY*() macros. These macros do not set Py_TPFLAGS_HAVE_STACKLESS_EXTENSION. Therefore slp_prepare_slots() gets called
   twice.

Fix: Set Py_TPFLAGS_HAVE_STACKLESS_EXTENSION only in slp_prepare_slots()

The allocated PyMappingMethods is never freed

No problem for static types, but it is a problem for heap types. We can free the memory in the tp_dealloc slot function of PyType_Type (Python class type). If Py_TPFLAGS_HAVE_STACKLESS_EXTENSION and Py_TPFLAGS_HEAPTYPE are set, tp_as_mapping was allocated by slp_prepare_slots()