Fix testing errors - which surfaced with pytest 6.0.1

freqtrade/exchange/exchange.py

@@ -258,8 +258,8 @@ class Exchange:
                 api.urls['api'] = api.urls['test']
                 logger.info("Enabled Sandbox API on %s", name)
             else:
-                logger.warning(name, "No Sandbox URL in CCXT, exiting. "
-                                     "Please check your config.json")
+                logger.warning(f"No Sandbox URL in CCXT for {name}, exiting. "
+                                "Please check your config.json")
                 raise OperationalException(f'Exchange {name} does not provide a sandbox api')
 
     def _load_async_markets(self, reload: bool = False) -> None: