BUG: Series.get() with ExtensionArray and integer index
- [x] closes #21257
- [x] tests added / passed
    - added example in tests/extension/base/getitem.py:test_get()
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
    - Put this in 0.23.1 for now

