gh-91670: Removes `SO` config variable in `sysconfig.py` (#91671)

* Removes SO config variable in sysconfig

Per @warsaw in https://github.com/python/cpython/issues/63754, this was
deprecated in Python 3.4 and was suggested for removal in Python 3.5.

* Add NEWS

* Update Misc/NEWS.d/next/Library/2022-04-18-15-23-24.gh-issue-91670.6eyChw.rst

Co-authored-by: Éric <merwok@netwok.org>

Co-authored-by: Barry Warsaw <barry@python.org>
Co-authored-by: Éric <merwok@netwok.org>