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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def validate_key(self, activation_key):
 """ Verify that the activation key is valid and within the permitted activation time window, returning the username if valid or ``None`` if not. """

        try:
            username = signing.loads(
                activation_key,
                salt=self.key_salt,
                max_age=conf.get('ACCOUNT_ACTIVATION_DAYS') * 86400
            )
            return username
        # SignatureExpired is a subclass of BadSignature, so this will
        # catch either one.
        except signing.BadSignature:
            return None