UB/segfault with unicode slice

```
cdef extern from *:
    cdef Py_ssize_t PY_SSIZE_T_MIN
    cdef Py_ssize_t PY_SSIZE_T_MAX

u""[PY_SSIZE_T_MAX:PY_SSIZE_T_MIN]
```

this should suffice to repro; this leads to overflow of a Py_ssize_t and a segfault at higher optimization levels, PR forthcoming