API/DEPR: replace kwarg "pat" with "sep" in str.[r]partition
- [x] closes #22676
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @h-vetinari! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/strings.py`](https://github.com/h-vetinari/pandas/blob/8cbf9eac7e3d20357e720ea9817a42e2fa72db3b/pandas/core/strings.py) !

 - There are no PEP8 issues in the file [`pandas/tests/test_strings.py`](https://github.com/h-vetinari/pandas/blob/8cbf9eac7e3d20357e720ea9817a42e2fa72db3b/pandas/tests/test_strings.py) !

 Failure is a flaky hypothesis test. looks fine, ex @WillAyd comments. @WillAyd pls add this to the list in #6581 as well. merge when ready. @jreback @WillAyd 
The feedback of @WillAyd is already incorporated. thanks @h-vetinari 