PR GH-27832 inadvertently (I believe) introduced a couple of changes to PyLong_FromLong that didn't make a lot of sense: an (unsigned long) cast was replaced with (twodigits), and a digit count variable (counting number of PyLong digits in a C long) had its type needlessly changed from int to Py_ssize_t.

The first change is a potential portability bug, but only on platforms where the width of C's `long` exceeds 64 bits. The (unsigned long) cast is obviously correct, while figuring out whether (twodigits) loses information takes some work.

The second change is merely a potential pessimization: there's no need to use what's typically a 64-bit integer to count the number of PyLong digits in a long.

While fixing these, I've also reworked the PyLong_FromLong code a bit to move the medium int check earlier, and I've applied analogous changes to PyLong_FromLongLong.

New changeset c60e6b6ad7aaf9c72035ff9fb1575e2710aeb4b4 by Mark Dickinson in branch 'main':
bpo-46311: Clean up PyLong_FromLong and PyLong_FromLongLong (GH-30496)
https://github.com/python/cpython/commit/c60e6b6ad7aaf9c72035ff9fb1575e2710aeb4b4