Migrate from aiohttp to asks

[mehaase]

aiohttp and aiosocks are the last asyncio libraries in the project, and both are used in the downloader. We could migrate aiohttp to asks, which is fully Trio native, but there is no Trio equivalent to aiosocks yet.

[mehaase]

Also while updating the downloader, I can refactor how the session is created. The session object is recreated for every download, but most of its contents are the same. I could either create a kwargs dict in the constructor, or maybe even instantiate the Session object itself.

[mehaase]

This looks like a potential solution: https://github.com/Ostoic/trio-socks