I forgot to say that this behavior was not present in stable version 3.8.5 . Sorry.

On 2 machines AIX 7.2, testing Python 3.8.5 with:
+ cd /opt/freeware/src/packages/BUILD/Python-3.8.5
+ ulimit -d unlimited
+ ulimit -m unlimited
+ ulimit -s unlimited
+ export LIBPATH=/opt/freeware/src/packages/BUILD/Python-3.8.5/64bit:/usr/lib64:/usr/lib:/opt/lib
+ export PYTHONPATH=/opt/freeware/src/packages/BUILD/Python-3.8.5/64bit/Modules
+ ./python Lib/test/regrtest.py -v test_decimal
...
gave:

507 tests in 227 items.
507 passed and 0 failed.
Test passed.

So, this issue with v3.10 (master) appeared to me as a regression. However, after hours debugging the issue, I forgot to say it in this defect, sorry.

(Previously, I was using limits for -d -m and -s : max 4GB. However, that appeared to be an issue when running tests with Python test option -M12Gb, which requires up and maybe more than 12GB of my 16GB memory machine in order to be able to run a large part of the Python Big Memory tests. And thus I unlimited these 3 resources, with no problem at all with version 3.8.5 .)