Vendor curio in tests? It is deprecated on PyPI

[webknjaz]

FYI dabeaz/curio@45ada85#diff-7b3ed02bc73dc06b7db906cf97aa91dec2b2eb21f2d92bc5caa761df5bbc168fR13-R17

https://twitter.com/dabeaz/status/1584922164880457728

[theoparis]

Since curio hasn't had a new release and the commit that fixes building on python 3.13 is only available in the master branch of curio, yt-dlp fails to build on NixOS with python 3.13.