> However, doing the check on 'other' and raising a TypeError
> with an appropriate message would still be better 

Let's be clear.  These are duck-typed methods.  A type check is inappropriate.  Anything with o.items() is allowed regardless of type.

Also, I generally won't approve changes to existing APIs without compelling real-world use cases to motivate the design change.  Otherwise, you just create unnecessary churn and consternation.