BUG: NumericIndex.append retain dtype
- [ ] 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
- [ ] whatsnew entry

can you add a whatsnew note, otherwise lgtm > can you add a whatsnew note, otherwise lgtm

shouldnt need whatsnew since NumericIndex isn't exposed in a released version