Convert ExchangeResolver to static loader class

freqtrade/optimize/backtesting.py

@@ -60,7 +60,7 @@ class Backtesting:
         # Reset keys for backtesting
         remove_credentials(self.config)
         self.strategylist: List[IStrategy] = []
-        self.exchange = ExchangeResolver(self.config['exchange']['name'], self.config).exchange
+        self.exchange = ExchangeResolver.load_exchange(self.config['exchange']['name'], self.config)
 
         if config.get('fee'):
             self.fee = config['fee']