At Objects/longobject.c, in almost all cases
OverflowError is raised when a unsigned integral is requested from a
negative PyLong. However, this one breaks the rules:

int
_PyLong_AsByteArray(PyLongObject* v,
                  unsigned char* bytes, size_t n,
                  int little_endian, int is_signed)
{
<...>
              if (!is_signed) {
                      PyErr_SetString(PyExc_TypeError,
                              "can't convert negative long to 
unsigned");
                      return -1;
              }
<...>
}