BUG: Try to sort result of Index.union rather than guessing sortability
- [x] closes #17376
- [x] tests added / passed
- [x] passes `git diff master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

The lines of code I removed seemed written with the intention of guaranteeing the same result under Python 2 and 3, but the docs say "Form the union of two Index objects and sorts _if possible._": so if it is possible in Python 2 we should do it, regardless of whether it is in Python 3. After all, this is how ``.sort_values()`` works. And more practically, trying to guess whether the result of an union can be sorted is hard.