Fix: StringArray non-extensible due to inconsisent assertion
Fix for #34309

- [X] closes #34309
- [X] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

@dsaxton Thanks for your review. I've processed your proposed changes and added an additional string dtype test. > Tiny nit, but otherwise LGTM

Done cc @jorisvandenbossche @TomAugspurger  I think the issue has an ongoing discussion that needs to be resolved.  @sbrugman can you merge master just to keep this from drifting Hello @sbrugman! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-09-23 21:22:44 UTC This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this.