BUG-24408 Series.dt does not maintain own copy of index
- [X] closes #24408
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

Previously, the `dt` accessor maintains its own copy of the index, so in-place operations would cause the indexes to be out of sync. This PR changes the accessor so it just gets the index from its parent.