If we do decide to do this, I'm inclined to eventually make the change at the Grammar level rather than the AST level.

Current:

    comparison: expr (comp_op expr)*
    comp_op: '<'|'>'|'=='|'>='|'<='|'<>'|'!='|'in'|'not' 'in'|'is'|'is' 'not'

Future:

    comparison: expr (comp_op expr)* | (containment_check)
    comp_op: '<'|'>'|'=='|'>='|'<='|'<>'|'!='|'is'|'is' 'not'
    containment_check: ('in'|'not' 'in') expr

However, we'd still need an intermediate step like your PR in order to emit SyntaxWarning while still retaining the current semantics.