[enh] #91 fix pylint alerts in documents.certification module

duniterpy/documents/certification.py

@@ -173,7 +173,7 @@ CertTimestamp: {timestamp}
         self.signatures = []
         for key in keys:
             signing = base64.b64encode(key.signature(bytes(self.raw(), 'ascii')))
-            logging.debug("Signature : \n{0}".format(signing.decode("ascii")))
+            logging.debug("Signature : \n%s", signing.decode("ascii"))
             self.signatures.append(signing.decode("ascii"))
 
     def signed_raw(self) -> str: