Signature.__repr__ already exits. It is inherited from object. So you must be proposing to override the inherited method. For what purpose? With what outcome?

Without reading the signature PEP, I believe the current difference between str and repr is intentional, not accidental. Example:
>>> repr(s)
'<inspect.Signature object at 0x02E7BC08>'
>>> str(s)
'(self, a)'

If so, this issue should be closed unless you can provide an excellent reason to break code the expects the current behavior.