Remove support for curio

[codypiersall]

curio releases on PyPI do not support Python 3.12—remove support for curio and move all examples to use trio instead.

dabeaz/curio#367

Hopefully this will also allow us to start building wheels for Python 3.13.

We don't plan to completely stop supporting curio, just stop testing it, since we cannot use the pytest-curio plugin.

[craftyjon]

@codypiersall is this something you have in progress yourself, or something you'd like help with?

[codypiersall]

@craftyjon Thanks for the offer! I've got a branch for it; once it passes CI I'm going to merge it, and then get 3.13 wheels built—shouldn't be too much longer now.