Consider switching to httpx over aiohttp

[Cadair]

I just discovered https://github.com/encode/httpx which looks simplier to work with, and has a lighter weight dependency stack than aiohttp. Might be worth investigating further.

[Cadair]

This came up again recently in the context of using parfive inside jupyterlite/pyiodide. pyiodide has a patched version of httpx which allows it to work inside emscripten.

I've been thinking about how to handle this migration a bit, I'm considering making the "transport" part pluggable, so that you can use aiohttp httpx or aioftp in the same code path which would simplify the code a bit by removing the http / ftp split.