BUG: DataFrame._item_cache not cleared on on .copy()
- [x] closes #31784
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

lgtm. merging pending doc-build merge (so nothing to do @neilkg ) thanks @neilkg 