Change special return value for REMOVE

[LWprogramming]

The visitor.py file sets the special value REMOVE to Ellipsis. However, this causes trouble with type-hinting functions that return REMOVE. Could we create a separate value rather than using Ellipsis? See the issues linked here, where the consensus is in favor of not using the Ellipsis constant. Thank you!

[Cito]

Maybe we should create an enum VisitorAction containing these four special values, and set REMOVE to VisitorAction.REMOVE etc. Then adding type hints would be even easier. For backward compatibility, the visitor could still recognize the other special values like Ellipsis.

Would that solve the problem?

[LWprogramming]

That would work! Arguably it would work even better because we wouldn't need to update the type-hint every time we have a visitor function return a different special value.

[Cito]

I have implemented this in the master branch now.