Pep8ify examples

This PR converts the animation, api and axes_grid examples.

(Retry of #3182/#3183)

I addressed @ianthomas23' comments from #3183
Interesting, but I think isort is probably overkill for us, and would result in unproductive code churn.  I think our imports are mostly OK.  (If it were to be used, my configuration choices would include "multi-line-output=0".)  I sympathize with the author's explanation at the end: "I'm too lazy to spend 8 hours mindlessly performing a function, but not too lazy to spend 16 hours automating it."