Remove fstring from place_stop_order to make compatible with python < 3.6

Author: Mike Cardillo

cbpro/authenticated_client.py

@@ -456,7 +456,7 @@ def place_stop_order(self, product_id, stop_type, price, size=None, funds=None,
         elif stop_type == 'entry':
             side = 'buy'
         else:
-            raise ValueError(f'Invalid stop_type for stop order: {stop_type}')
+            raise ValueError('Invalid stop_type for stop order: ' + stop_type)
 
         params = {'product_id': product_id,
                   'side': side,