PEP 484: Fix upper bound example (#1421)

The example in the PEP was incorrect: it does not work since it would require `Comparable` to be defined as a Protocol, which were introduced in a later PEP (PEP 544).

For more info see #1419.

pep-0484.txt

@@ -722,32 +722,21 @@ Type variables with an upper bound
 A type variable may specify an upper bound using ``bound=<type>`` (note:
 <type> itself cannot be parameterized by type variables). This means that an
 actual type substituted (explicitly or implicitly) for the type variable must
-be a subtype of the boundary type. A common example is the definition of a
-``Comparable`` type that works well enough to catch the most common errors::
+be a subtype of the boundary type. Example::
 
-  from typing import TypeVar
+  from typing import TypeVar, Sized
 
-  class Comparable(metaclass=ABCMeta):
-      @abstractmethod
-      def __lt__(self, other: Any) -> bool: ...
-      ...  # __gt__ etc. as well
+  ST = TypeVar('ST', bound=Sized)
 
-  CT = TypeVar('CT', bound=Comparable)
-
-  def min(x: CT, y: CT) -> CT:
-      if x < y:
+  def longer(x: ST, y: ST) -> ST:
+      if len(x) > len(y):
           return x
       else:
           return y
 
-  min(1, 2)      # ok, return type int
-  min('x', 'y')  # ok, return type str
-
-(Note that this is not ideal -- for example ``min('x', 1)`` is invalid
-at runtime but a type checker would simply infer the return type
-``Comparable``.  Unfortunately, addressing this would require
-introducing a much more powerful and also much more complicated
-concept, F-bounded polymorphism.  We may revisit this in the future.)
+  longer([1], [1, 2])  # ok, return type List[int]
+  longer({1}, {1, 2})  # ok, return type Set[int]
+  longer([1], {1, 2})  # ok, return type Collection[int]
 
 An upper bound cannot be combined with type constraints (as in used
 ``AnyStr``, see the example earlier); type constraints cause the