[hitomi] applying avif check for every image (#3030)

[styler00dollar]

A webp fallback was introduced after I made aware of it in a github issue, but it does not check every image. As an example, an image sequence like avif, webp, avif, webp, webp, avif, would download incorrectly, or rather, skip all webp due to broken links. The first image is avif, it passes and then tries to download everything as avif, but avif does not exist for every image. An avif property check for every image is needed, since not all files have the same file format in one gallery.

[styler00dollar]

I did not notice that i need to check formatting with flake8.