Use of `AttrsInstance` protocol is incompatible with type checkers other than mypy

The latest release introduced the AttrsInstance protocol with one attribute, __attrs_attrs__, which replaced Any in the signature of various core functions. The __attrs_attrs__ attribute is "injected" by the attrs mypy plug-in; other type checkers have no knowledge of this attribute and adding attributes to the type definition of a dataclass-like class is not supported by dataclass_transform. Therefore, functions such as fields and asdict generate type errors by default in all type checkers other than mypy. I am not familiar with mypy plug-ins but I assume one alternative would be to remove the attribute from the protocol definition and inject it dynamically from the plug-in, the way it's done for the attrs class. Longer term, an extension to dataclass_transform could be sought.

Start by reproducing the type errors in fields and asdict with type checkers other than mypy, then read the AttrsInstance protocol and the attrs mypy plug-in behavior. Compare removing attrs_attrs from the protocol with injecting it through the plug-in; done means these core functions type-check by default outside mypy without breaking the attrs plug-in path.