any reason why ExtendedTraverserVisitor does not set allow_interpreted_subclasses=True?

Feature

I got a TypeError: interpreted classes cannot inherit from compiled when I tried to extend from it. Not sure if this is a bug or not.

To reproduce:

from mypy.parse import parse
from mypy.traverser import ExtendedTraverserVisitor

class FooVisitor(ExtendedTraverserVisitor):
    def visit(self, o: Node) -> bool:
        print("hello")
        return True

parse("print('foo')", None, None, None, Options()).accept(FooVisitor())

Start by reading mypy/traverser.py, especially ExtendedTraverserVisitor, and mypy/parse.py to understand how the parsed tree reaches the visitor. Reproduce the TypeError with the example, then determine whether enabling interpreted subclasses is intended and verify the chosen behavior with a focused test or documented rationale.