BUG: Fix Series.get() for ExtensionArray and Categorical
- [x] closes #20882
- [x] tests added / passed
  - pandas/tests/extension/base/getitem.py::test_get()
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
  - Not created - assume will go in v0.23 as part of ExtensionArray support
