Adding runtime_checkable to AttrsInstance

[ryan-minato]

Is it possible to add the runtime_checkable decorator to AttrsInstance protocol?

attrs/src/attr/__init__.py

Lines 37 to 38 in 598494a

	class AttrsInstance(Protocol):
	pass

In my project, I am working on a decorator that automates converting a dict into an AttrsInstance.
To perform reflection checks on input definitions, I found myself needing to subclass AttrsInstance without modifying it—just to add the runtime_checkable decorator.
It would be incredibly helpful if the attrs library's definition already included runtime_checkable.

[hynek]

As @Tinche commented on your PR, this is not trivial, because runtime_checkable checks whether an instance fulfills the Protocol you defined and since AttrsInstance is empty, every class will conform.

Maybe it's possible if we switch to an abc with a dedicated __instancecheck__?