Fix return type of Timeline.__iter__

[domsmrz]

Small fix of provided typing. Timeline.__iter__ already returns iterator (not just iterable) as required -- This change propagates this information to the actual annotation.

[hbredin]

This is some code I wrote some long time ago and I am not quite sure about the difference between Iterable and Iterator.

Also, would it make sense to stick to Iterable and remove the call to iter?

[domsmrz]

I believe the proposed change is good as-is:

• Iterator is required to have __next__, so it can be passed into next method (as in next(my_iterator)). It can be somewhat thought of as a pointer to a specific place inside a list or another container (and the next method is a way to make the pointer move to the next element).
• Iterable on the other hand is required to have __iter__ method. The __iter__ method should return Iterator (via which one can iterate over the Iterable). Classical example of Iterables are lists.

Additionally, IIRC Iterators should also implement __iter__ method, meaning every Iterator is also Iterable, but that is not really important here.

Hence, the iter call at the end of the method is required (otherwise the method would return Iterable and not Iterator but __iter__ has to return Iterator, otherwise -- for example -- for loops wouldn't work).

[hbredin]

Thanks!