Fix base class for correct typing support

[Serhii Tereshchenko]

Hi, we're using pyright to type-check our code, and djangorestframework-types as types for DRF.

Type for filter_backends is defined as

    filter_backends: Sequence[Type[BaseFilterBackend]]

So, all filter must be subclasses of BaseFilterBackend.

Could you please add it as base class to DjangoFilterBackend?

[Carlton Gibson]

It might be that we can define a Protocol here, but adding a inheritance just to satisfy the type checker is not the right move.

[Serhii Tereshchenko]

yeah, we can definitely add protocol. Thanks.