`AttrsInstance` protocol in the new `attrs` namespace

[tsx]

AttrsInstance protocol was added in #890

However attrs.asdict and attrs.astuple still take Any as the instance. This reduces type safety for people using the "modern" API. Was there any reason for these two functions being left out of the original AttrsInstance change?

[Tinche]

I think we just forgot. A good opportunity for a quick PR!

[tsx]

Opened a quick PR.