> There would still be a breakage for that if someone was defining py36+ `visit_Constant` (which would clobber the `ast.NodeVisitor.visit_Constant` if we were to add it)


Ok, it would still break in some cases, but that would still be a net improvement for codebase that do not override visit_Constant; right ? 

We could also push the patch further if we really want and call explicitly NodeVisitor.visit_Constant before the overridden method in NodeVisitor.visit.

Not advocating for the second part, but minimal breakage and code repetition in libraries when we can.