Attributes which are qualified as Final may not be overwritten in subclasses (spec):

from typing import Final

class Base:
    CONSTANT: Final[int] = 1

class Derived(Base):
    CONSTANT = 2  # This should be an error

We have an existing test (with TODOs) for this here.

I imagine that it might be convenient to implement this once we have #166 (because that would require similar mechanisms to look up equivalent attributes in base classes), but it might also make sense to implement this independently.