Fix python3 issues in some examples

Unfortunately these are the easy ones ...

This mostly involves intervals/keys and xrange. For the most part I chose simplicity over minimising memory usage and replaced interkeys etc. with keys. The loops are rather short so the benefits from iterators is rather limited. If anyone thinks it is important I am happy to change this.
I deliberately chose to avoid six as much as possible to simplify the examples. As I see it examples has a different balance between code readability and performance. But you are probably right that in the cases of axes spines etc. which are somewhat longer loops it makes sense to use iterators.