Support broticffi in addition to brotli

[mgorny]

aiohttp currently supports only the brotli bindings for Brotli. Unfortunately, these stopped working on PyPy recently and, well, we haven't been really able to figure out what's wrong. So far the only suggestion was to use brotlicffi instead (which might be better suited for PyPy anyway).

Would you be interested in adding support for brotlicffi alternatively to brotli? I can make a pull request. Just let me know whether you prefer the "fallback" approach (i.e. trying brotli, then brotlicffi on ImportError), or switching between brotli and brotlicffi depending on whether CPython or PyPy is being used.

TIA.

[Dreamsorcerer]

I'm not sure how it's implemented currently, so not sure what approach is possible.
But, I think best approach would be to have an easy method to plug in any custom library, if that's possible (as we've just done with charset guessing in the upcoming 3.9).
If that's not possible, then probably what is recommended on their readme: https://pypi.org/project/brotlicffi/#using-brotlicffi-in-projects
(I started to write my thoughts down, and then looked at their readme, and it recommended exactly what I was thinking).

[mgorny]

Thanks. I'm going to try the latter approach, and try to do minimal changes to the existing code.