Modernize code to Python 3.6+ and some cleanup

10-dp-1class-func/strategy_best3.py

@@ -8,7 +8,7 @@
     >>> ann = Customer('Ann Smith', 1100)
     >>> cart = [LineItem('banana', 4, .5),
     ...         LineItem('apple', 10, 1.5),
-    ...         LineItem('watermellon', 5, 5.0)]
+    ...         LineItem('watermelon', 5, 5.0)]
     >>> Order(joe, cart, fidelity_promo)
     <Order total: 42.00 due: 42.00>
     >>> Order(ann, cart, fidelity_promo)
@@ -73,8 +73,7 @@ class Order:  # the Context
         return self.total() - discount
 
     def __repr__(self):
-        fmt = '<Order total: {:.2f} due: {:.2f}>'
-        return fmt.format(self.total(), self.due())
+        return f'<Order total: {self.total():.2f} due: {self.due():.2f}>'
 
 
 # tag::STRATEGY_BEST3[]