gh-122404: Fix reference leak in sum implementation

[Eclips4]

• Issue: test_builtin leaks references #122404

[Eclips4]

cc @skirpichev

[Eclips4]

With this fix applied, test_builtin no longer leaks references:

eclips4@HOME-PC ~/p/cpython (issue-122404)> ./python -m test -R 3:3 test_builtin
Using random seed: 3525955014
0:00:00 load avg: 33.00 Run 1 test sequentially in a single process
0:00:00 load avg: 33.00 [1/1] test_builtin
beginning 6 repetitions. Showing number of leaks (. for 0 or less, X for 10 or more)
123:456
XX. ...

== Tests result: SUCCESS ==

1 test OK.

Total duration: 11.8 sec
Total tests: run=135 skipped=5
Total test files: run=1/1
Result: SUCCESS

[Eclips4]

Closing per #122234 (comment)