Pep8ify examples

This PR converts the animation, api and axes_grid examples.

(Retry of #3182/#3183)

I addressed @ianthomas23' comments from #3183
@efiring thx for your comment. I care a lot about consistency and therefore want to help making the mpl codebase more consistent. There already exists a tool to break a list of imports into multiple imports called isort (https://github.com/timothycrosley/isort - check out its Readme file). Automating fixing your other mentioned issues is probably not worth the effort even if I fully agree to your comments.