Added FutureWarning to Series.str.__iter__
- [x] #28277
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

@jreback The deprecation follows from the discussion in #28319 where an actual fix to the issue was suggested:

WillAyd:
> I think the best approach would be to deprecate the __iter__ method of Series.str as noted above and remove in the future > @jreback The deprecation follows from the discussion in #28319 where an actual fix to the issue was suggested:
> 
> WillAyd:
> 
> > I think the best approach would be to deprecate the **iter** method of Series.str as noted above and remove in the future

ok i see the conversation and agree deprecation is ok. but this doesnt' fix the actual bug #28277 
 right? (meaning that should be a separate PR). Yes, the bug isn't fixed by that, but @TomAugspurger suggested (in the issue) the expected behaviour should be to raise an exception:
> series.str is an accessor, not an array-like of strings. I would expect an exception to be raised here.

So even though the bug isn't fixed immediately, in later versions, when this deprecation is taking effet, it will be resolved. Do you mean we should keep the issue open until then @jreback? Resolved merge conflict and removed the "closes" tag from the PR description @jreback  @SaturnFromTitan can you merge master CI is green @jreback  great will look soon thanks @SaturnFromTitan 