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

[kawochen]

closes #11108

[jreback]

why is this bad? e.g. if you are iterating with a generator with works just fine

[kawochen]

this causes a PendingDeprecationWarning in 3.5 and will be deprecated in 3.7 (PEP479)
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)

[jreback]

is this an API change though?

[kawochen]

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.

[jreback]

ok seems reasonable. add a whatsnew note.

[kawochen]

which section?

[jreback]

I guess API changes; this is mainly to have a record of this.

[kawochen]

green

[jreback]

thanks!