ENH: IntervalArray.min/max
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

looks fine, can you merge master and ping on green rebased + green-ish (same lint complaints as on master)