Modernize code to Python 3.6+ and some cleanup

06-obj-ref/bus.py

@@ -1,4 +1,3 @@
-
 """
 >>> import copy
 >>> bus1 = Bus(['Alice', 'Bill', 'Claire', 'David'])

06-obj-ref/cheese.py

@@ -2,7 +2,7 @@
 >>> import weakref
 >>> stock = weakref.WeakValueDictionary()
 >>> catalog = [Cheese('Red Leicester'), Cheese('Tilsit'),
-...                 Cheese('Brie'), Cheese('Parmesan')]
+...            Cheese('Brie'), Cheese('Parmesan')]
 ...
 >>> for cheese in catalog:
 ...     stock[cheese.kind] = cheese
@@ -24,5 +24,5 @@ class Cheese:
         self.kind = kind
 
     def __repr__(self):
-        return 'Cheese(%r)' % self.kind
+        return f'Cheese({self.kind!r})'
 # end::CHEESE_CLASS[]