Fix dict when validate assignment true

[austinpgraham]

Issue reported here.

This PR fixes an issue with v1.1.0 where setting validate_assignment = True on a Schema throws an Attribute error for dict on DjangoGetter. Example, from added test with change removed to force the error:

FAILED tests/test_schema.py::test_django_getter_validates_assignment - AttributeError: 'DjangoGetter' object has no attribute '__dict__'

It appears this functionality was broken when GetterDict was removed as a parent from DjangoGetter, and adding dict in slots seems to be the easiest fix.

I'm very open to feedback, and if this is intended or what a better fix might be, this PR is just me taking a stab at fixing something needed by my company.

Note: It appears running the linting fixed a couple other linting issues unrelated to this change.

[vitalik]

Hi @austinpgraham

Thank you

could you reformat some code-style changes with

pip install -U ruff
ruff format ninja tests

[austinpgraham]

Done, thanks @vitalik

[sandro-encora]

@vitalik Any plans on releasing a new version with this fix? Thank you!

[bastiaan85]

@vitalik +1 for me too