Remove @builtinclass

[JukkaL]

The PEP 484 draft doesn't have @builtinclass, so remove it. Alternatively, figure out an alternative way of encoding this information.

[elazarg]

If I am not mistaken, it can be removed right now. No test demonstrate a need for it.

The only use of this information inside mypy (through ClassDef.is_builtinclass) is in nearest_builtin_ancestor(), called from check_multiple_inheritance() in checker.py[1]. Replacing

-        if base.defn.is_builtinclass:
+        if base.defn.fullname == 'builtins.object':

passes all the tests[2], except the whitebox-test testBuiltinclassDecorator in semanal-classes.test, but this test only verifies that the implementation tracks this information.

[1] It resides on meet.py, for some reason, adding an unnecessary dependency
[2] Otherwise it could be replaced with looking up the decorator. Or keeping a list of builtin classes

[gnprice]

Thanks for investigating! Sounds like a small patch with that cleanup would be great.

[gvanrossum]

Let's leave this open until it's also removed from typeshed (two lines).