Use old style string formatting

So we don't break Python 2 quite yet

duo_client/client.py

@@ -310,7 +310,7 @@ def api_call(
                 body = ''
                 params = normalize_params(params)
         else:
-            raise ValueError(f"unsupported sig_version {sig_version}")
+            raise ValueError('unsupported sig_version {}'.format(sig_version))
 
         if self.sig_timezone == 'UTC':
             now = email.utils.formatdate()