Upstreaming stubs for `aiofiles`

[nenb]

Typeshed currently hosts stubs for aiofiles: https://github.com/python/typeshed/tree/main/stubs/aiofiles

I asked aiofiles if they'd be happy to inline the stubs, and it looks like they would: Tinche/aiofiles#49 (comment)

I've given this a go here: Tinche/aiofiles#222

Please feel free to leave comments or suggestions in the PR! (I've basically just vendored the stubs from typeshed, and add some basic CI scripting to get everything to run)