CLN/COMPAT: Remove raise StopIteration syntax in SparseArray.__iter__
closes #11108 

why is this bad? e.g. if you are iterating with a generator with works just fine
 this causes a `PendingDeprecationWarning` in 3.5 and will be deprecated in 3.7 ([PEP479](http://legacy.python.org/dev/peps/pep-0479/))
this syntax doesn't cause a problem here, but it could if combined with a context manager, where the `StopIteration` exception will be caught in `__exit__()` and potentially causes the context manager to take a different path (depending on the presence of try-except constructs in the context manager)
 is this an API change though?
 No.  Using `return` (or just leaving it out, since there is no more code) and `raise StopIteration` are the same in this function (except for the warnings).  They are different only when there is something inside the generator that could interfere with exception propagation.
 ok seems reasonable. add a whatsnew note.
 which section?
 I guess API changes; this is mainly to have a record of this.
 green
 thanks!
