Class helper functions - Support for generic classes

[LukasKrocek]

I noticed that functions like attr.has, attr.fields, attr.fields_dict doesn't work as i expected when i pass in class with generic type defined:

from attr import frozen, evolve, mutable, has, fields, fields_dict, Attribute

T = TypeVar("T")

@frozen
class A(Generic[T]):
    a: int
    b: T

has(A) => True
has(A[str]) => False (should be True)

fields(A) => Sequence[Attribute]
fields(A[str]) => TypeError: Passed object must be a class (should be Sequence[Attribute])

fields_dict(A) => Mapping[str, Attribute]
fields(A[str]) => TypeError: Passed object must be a class (should be Mapping[str, Attribute])```

[Tinche]

Will be taking a look at this soon.

[fredsensibill]

+1

This is quite annoying when I'm using cattrs' hook factories... e.g. the snake_case to camelCase cattrs example doesn't work with generic classes

[hynek]

first support has been merged as part of #1079