Remove `SO` config variable in `sysconfig.py`

[bryanwweber]

Feature or enhancement

In bpo-19555 (#63754), @warsaw deprecated the SO configuration variable in sysconfig.py and suggested it could be removed after Python 3.5. It seems that never happened. I looked for other issues and discussions, but didn't find anything since the original report. Please let me know if I missed something!

Per suggestion from @warsaw, I'm creating this issue following a discussion on discuss.python.org

Pitch

Removing deprecated code seems like a Good Thing™️. This code has been deprecated for almost 10 years and 7 Python releases.

Previous discussion

• "SO" config var not getting set #63754
• https://discuss.python.org/t/remove-so-config-variable-in-sysconfig-py/15142