feat: define default visit method for visitor

[bolajiwahab]

Description

This change fixes the typing inconsistency in the visitor base class by replacing the dynamically assigned visit = None attribute with a properly defined method signature.

Previously, the base class exposed visit as an attribute:

visit = None

while subclasses can either choose to implement/override it as a concrete method:

def visit(self, ancestors: visitors.Ancestor, node: ast.Node) -> None:

This then creates a divergence between the superclass and subclass definitions:

• superclass treats visit as an optional attribute/callback
• subclasses treated visit as an actual overridable method

Although this worked at runtime due to Python’s dynamic nature, static type checkers such as mypy interpreted the superclass contract differently and reported errors similar to:

Signature of "visit" incompatible with supertype "Visitor"  [override]

because the superclass did not define a real method signature to override.

This PR resolves the issue by defining visit() as an explicit no-op method in the base class, establishing a consistent and type-safe inheritance contract for subclasses.

[lelit]

Ok, please remove this last sentence from the docstring, and check the call site to remove the is None protection.

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Additional details and impacted files

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[lelit]

Thank you, and sorry for being so fussy. I can take care of these last minutiae myself.