CLN: move maybe_casted_values from pandas/core/frame.py to pandas/core/dtype/cast.py
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

xref #36876, #27370 (stale PR)