Update Model subclass DoesNotExist type

[mateenkasim]

I have made things!

• Updates stub for DoesNotExist error on model subclass
• In VSCode, Pylance considers all DoesNotExist errors to be the same error, when the reality is that each model has its own independent DoesNotExist. This is discussed in the referenced issue on Pylance's GH, where they attribute it to the stub. This PR updates the stub using their suggestion.

try:
    # some django code
except ModelA.DoesNotExist:
    # handle ModelA error
except ModelB.DoesNotExist:
    # pylance thinks this code is unreachable before this PR

Related issues

• Refs: Unreachable code for common django exception microsoft/pylance-release#2770

Notes

• In all honesty, I don't truly understand stubs – I just copied the suggestion from the folks at Pylance + formatting described in the contribution guide
• Formatting and testing went through smoothly, except for tests/typecheck/models/test_ineritance.yml/django_contrib_gis_base_model_mixin_inheritance, which errored because I don't have something called GDAL installed? Let me know if I truly caused this error, or if it's just a machine-specific thing.

[mschoettle]

Cool! Does it make sense to make it Final at the same time? I don't think it is meant to be overridable, is it?

[mateenkasim]

I think that makes sense – a DoesNotExist error has one clear purpose, as opposed to something like a save function which is often overridden.

I left the Final out because it's not used much in this repo, but I'm happy to make that change!

[mschoettle]

Thanks, LGTM!

[intgr]

Thanks!