BUG: Fix interval constructor to handle left=right case
- [X] closes #26893
- [] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

**Update:**

[ ] - Following https://github.com/pandas-dev/pandas/pull/26894#pullrequestreview-250742121, needs tests about creating an `Intervalindex` with '(x,x]', also checking that the repr doesn't blowup.