REGR: inplace Series op not actually operating inplace
- [x] closes #36373
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

alternative to #37497, ensures that Series inplace ops are actually inplace whenever possible.  The whatsnew is copied verbatim from #37497, the new test is copied with an additional assertion about `series._values`