gh-115391: Fix "comparison of integers of different signs" warning in longobject.c

[sobolevn]

I think it is safe to just cast unsigned value to signed here, with minimal overflow risk, because we define cv.v = _PyLong_CompactValue(v);

CC original author and reviewers.

• Issue: New warning: "comparison of integer expressions of different signedness: ‘Py_ssize_t’ {aka ‘long int’} and ‘long unsigned int’ [-Wsign-compare]" #115391

[sobolevn]

Thanks! But, #115368 was first :)
I've missed it, but I will let him do that.