enable the cython binding=True directive

[graingert]

it becomes the default in cython3 and is needed for sniffio to detect uvloop as asyncio efficiently

[graingert]

Hmm, it might be worth updating to cython 3 instead?

[fantix]

Oh let me try that.

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OK, I'd probably do Cython 3 in another uvloop release, it'll take some time to adapt to Cython 3, also Cython 3 would be more tested at that time I think.