bpo-24076: Inline single digit unpacking in the integer fastpath of sum()

[scoder]

This gave me a 17% speedup last time I tried (3 years ago). Most integers that go into sum() should fit into a single PyLong digit, so this seems worth it.

https://bugs.python.org/issue24076

[pablogsal]

Unfortunately commit debd804 introduced a reference leak:

❯ ./python -m test test_grammar -R :
0:00:00 load avg: 2.96 Run tests sequentially
0:00:00 load avg: 2.96 [1/1] test_grammar
beginning 9 repetitions
123456789
.........
test_grammar leaked [12, 12, 12, 12] references, sum=48
test_grammar failed (reference leak)

== Tests result: FAILURE ==

1 test failed:
test_grammar

Total duration: 1.1 sec
Tests result: FAILURE

debd804 is the first bad commit
commit debd804
Author: scoder stefan_ml@behnel.de
Date: Tue Sep 21 11:01:18 2021 +0200

[bpo-24076](https://bugs.python.org/issue24076): Inline single digit unpacking in the integer fastpath of sum() (GH-28469)

.../Core and Builtins/2021-09-20-10-02-12.bpo-24076.ZFgFSj.rst | 1 +
Python/bltinmodule.c | 10 +++++++++-
2 files changed, 10 insertions(+), 1 deletion(-)
create mode 100644 Misc/NEWS.d/next/Core and Builtins/2021-09-20-10-02-12.bpo-24076.ZFgFSj.rst

[pablogsal]

Opened #28493 to fix the refleak