Add InlineModelAdmin.has_add_permission definition

[JoaRiski]

Add a proper definition for the InlineModelAdmin's has_add_permission method.

Fixes #469

NOTE: This change is entirely untested, so take extra care when reviewing. I'm unsure if the type check works in stub inheritance the same way it does in actual code, but if yes it could be this change will hit the same "incompatible with supertype" error as mentioned in #469

[blueyed]

LGTM.

The difference with InlineModelAdmin is that the parent object gets passed in there.

https://github.com/django/django/blob/742d6bc8dbc3868201895933e1c40c36f46462c7/django/contrib/admin/options.py#L2087
https://github.com/django/django/blob/742d6bc8dbc3868201895933e1c40c36f46462c7/django/contrib/admin/options.py#L2170-L2177

Can you rebase it to re-trigger CI?

[JoaRiski]

Looks like this needs to entirely remove the function from BaseModelAdmin and implement the ModelAdmin specific variant directly to ModelAdmin, as signature changes in subclasses don't seem to be allowed.

Would that sound reasonable?

[JoaRiski]

@blueyed made the change proposed above, still untested (just used GitHub's editor), but I don't think there should be an issue unless BaseModelAdmin is used in unexpected places.

Care to re-trigger the workflow?

[sobolevn]

Or maybe we can use # type: ignore on Inline's version of this method.

I think, that it will be more accurate. In this case, subclasses of BaseModelAdmin would still have has_add_permission method.

[JoaRiski]

I haven't checked how inheritance works with # type: ignore in the base class. Does that approach allow us to still provide proper type hints for inheritance, or would it make essentially just untyped?

[sobolevn]

Mypy copies the method definition from parent. It should be typed as we define it. But, we just silence the error there.