Return `node`s representation as slice.

def visit_Slice(self, node: AST, dfltChaining: bool = True) -> str:
        """Return `node`s representation as slice."""
        elems = [self.visit(node.lower), self.visit(node.upper)]
        if node.step is not None:
            elems.append(self.visit(node.step))
        return ':'.join(elems)