remove curio references

[belm0]

anyio 3.x does not support curio, hence asks doesn't

these refs should be removed

https://github.com/theelous3/asks/search?q=curio&type=code

[RaitoBezarius]

Furthermore, curio became harder to package as per https://github.com/dabeaz/curio#important-notice-october-25-2022.