BUG: wrong exception when slicing on TimedeltaIndex
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

inching towards sharing _maybe_cast_slice_bound between DTI/PI/TDI
release note? @meeseeksdev backport 1.3.x Something went wrong ... Please have  a look at my logs.