Add a new option for `stubtest`: `--ensure-runtime-generic`

[sobolevn]

In django-stubs we have a problem: some types that we declare generic are not generic in runtime.

We have a special package to monkeypatch runtime types to add __class_getitem__ to classes: https://github.com/typeddjango/django-stubs/blob/08cdfe576d62fc3a2dfc33197a3260cb0a0dc097/ext/django_stubs_ext/patch.py#L56

But, we cannot really catch this. We have a lot of report about some type being generic in stubs but not in runtime. And we have to add new entries to our list of patched types.

We would really love to see --ensure-runtime-generic option for stubtest which will check that types that are generic in stubs are also generic in runtime.

It should be switched off by default.

I can work on this if others are fine with the idea itself.