BUG: idxmax/min (and argmax/min) for Series with underlying ExtensionArray
- [x] closes #32749, closes #33719, closes #36566
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Currently `pd.Series([1, 2, 3]).astype('Int8').idxmax()` raises an error.
This request proposes a fix.
My first time using Github and opening a pull request. Apologise if it's no good.