Pep8ify examples

This PR converts the animation, api and axes_grid examples.

(Retry of #3182/#3183)

I addressed @ianthomas23' comments from #3183
It seems to be an idiosyncrasy of the particular pep8 tool that it always puts spaces around plus and minus, and never allows them around asterisk and solidus. This is completely contrary to the letter and spirit of the actual PEP 8.  Nevertheless, it seems we are stuck with it because the advantage of automated checking is deemed to outweigh the disadvantage of the occasional poor choice made by the automated tools. (In the line above, having spaces around the second asterisk would slightly enhance readability.)