BUG-20629 allow .at accessor with CategoricalIndex
- [X] closes #20629
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

This PR alters exception handling in various `get_value` functions to make calls to `.at` on non-integer `CategoricalIndex` fall back on positional indexing methods instead of raising exceptions, allowing queries like those mentioned in #20629.