Prohibit type variables inside ClassVar (#254)

Prohibit type variables inside TypeVar

pep-0526.txt

@@ -229,6 +229,10 @@ Here ``ClassVar`` is a special class defined by the typing module that
 indicates to the static type checker that this variable should not be
 set on instances.
 
+Note that a ``ClassVar`` parameter cannot include any type variables, regardless
+of the level of nesting: ``ClassVar[T]`` and ``ClassVar[List[Set[T]]]`` are
+both invalid if ``T`` is a type variable.
+
 This could be illustrated with a more detailed example. In this class::
 
   class Starship: