Look up certificate by public key, returns zcert_t object if found,
else returns NULL. The public key is provided in Z85 text format.

def lookup(self, public_key):
        """
        Look up certificate by public key, returns zcert_t object if found,
else returns NULL. The public key is provided in Z85 text format.
        """
        return Zcert(lib.zcertstore_lookup(self._as_parameter_, public_key), False)