Explicitly close loop in async tests

freqtrade/exchange/exchange.py

@@ -205,6 +205,8 @@ class Exchange:
                 and self._api_async.session):
             logger.debug("Closing async ccxt session.")
             self.loop.run_until_complete(self._api_async.close())
+        if self.loop and not self.loop.is_closed():
+            self.loop.close()
 
     def validate_config(self, config):
         # Check if timeframe is available