Forbid to use or register ClassVar

[sobolevn]

Refs #2978
Refs #3201

I think that duplication is now pretty visible. Should I do something about it? 🤔

[sobolevn]

I can't understand why mypy fails. The error message seems unhelpful.

[sobolevn]

Turns out, I've just missed this tiny line in the output 😆

[Zac-HD]

Looks good to me!

[Zac-HD]

I think that duplication is now pretty visible. Should I do something about it? 🤔

Yeah, at this point there seems to be a clear pattern. Next time (assuming it still makes sense), let's refactor to something like

for name, type_tuple in types.NON_RUNTIME_TYPES.items():
    if custom_type in type_tuple :
        raise InvalidArgument(
            f"custom_type={custom_type!r} is not allowed to be registered, "
            f"because there is no such thing as a runtime instance of {name}"
        )