Pep8ify examples

This PR converts the animation, api and axes_grid examples.

(Retry of #3182/#3183)

I addressed @ianthomas23' comments from #3183
@thisch I don't want to be too fussy--many things are a matter of taste, and not worth fiddling with--but if you are making extensive changes to improve style and make things more consistent with mpl guidelines, then you could also try to eliminate the use of trailing backslashes as line continuations.  Methods include the use of parentheses to make a list of imports, or breaking an import up into multiple imports; use of a temporary variable for a chunk of an expression that is otherwise getting out of hand; and taking advantage of whatever form of parentheses or brackets is appropriate.