Use __future__.annotations

[zrquan]

Use the new typing style, just like in httpx.

[tomchristie]

Okay. I do think they're more readable, so you're welcome to escalate this into an issue. Please include an example in the description.

[karpetrosyan]

I think it will be also useful to add a CI check, that will enforce us using the new python annotations, otherwise we should periodically update the old ones