Clean up PyLong_FromLong and PyLong_FromLongLong

[mdickinson]

BPO	46311
Nosy	@mdickinson, @markshannon
PRs	bpo-46311: Clean up PyLong_FromLong and PyLong_FromLongLong #30496

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2022-03-04.14:39:02.191>
created_at = <Date 2022-01-09.11:55:44.844>
labels = ['3.11']
title = 'Clean up PyLong_FromLong and PyLong_FromLongLong'
updated_at = <Date 2022-03-04.14:39:02.191>
user = 'https://github.com/mdickinson'

bugs.python.org fields:

activity = <Date 2022-03-04.14:39:02.191>
actor = 'Mark.Shannon'
assignee = 'none'
closed = True
closed_date = <Date 2022-03-04.14:39:02.191>
closer = 'Mark.Shannon'
components = []
creation = <Date 2022-01-09.11:55:44.844>
creator = 'mark.dickinson'
dependencies = []
files = []
hgrepos = []
issue_num = 46311
keywords = ['patch']
message_count = 2.0
messages = ['410141', '414263']
nosy_count = 2.0
nosy_names = ['mark.dickinson', 'Mark.Shannon']
pr_nums = ['30496']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue46311'
versions = ['Python 3.11']

[mdickinson]

PR #72019 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.

[markshannon]

New changeset c60e6b6 by Mark Dickinson in branch 'main':
bpo-46311: Clean up PyLong_FromLong and PyLong_FromLongLong (GH-30496)
c60e6b6