Implement ModelSerializer.__class_getitem__

[antonagestam]

To allow type checking efforts to properly type the methods of ModelSerializer it needs to be made generic with regards to the model it serializes. Since django-rest-framework has chosen to not own any annotations themselves the only way to achieve that will be to implement __class_getitem__. Stub implementors can then make the class generic and calling ModelSerializer[Author] will not yield subscriptability errors at runtime.

Note that this is the unfortunate direction Django has chosen as well.

This is the relevant issue in djangorestframework-stubs for reference.

This is the relevant Python docs on generic classes.

[sobolevn]

@antonagestam thanks a lot! This will make development of djangoresframework-stubs much easier! 👍