BUG: Fix Series.update ExtensionArray issue
- [x] closes #33980
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

@jreback ping thanks @dsaxton  hmm, the tests added here fail when backporting this fix. There have been many refactors of the indexing code since 1.0.3 > hmm, the tests added here fail when backporting this fix. There have been many refactors of the indexing code since 1.0.3

removing the needs backport label for now. @simonjayhawkins generally in backport i would do the least possible ones to avoid new issues this PR did fix a regression https://github.com/pandas-dev/pandas/issues/34020#issuecomment-624591164

so was a candidate for backport.  i understand but my point is that we only want backport a very minutes number of tegressions - esp later in a release series