Unsigned type can be used for more efficient checking on integer overflow.

    new_allocated = (newsize >> 3) + (newsize < 9 ? 3 : 6);
    new_allocated += (size_t)newsize;
    if (new_allocated < (size_t)newsize) {
        PyErr_NoMemory();
        return -1;
    }

Checking "new_allocated < (size_t)newsize" can be more efficient than "new_allocated > PY_SSIZE_T_MAX - newsize".