Allow deriving NewType from another unique type (#271)

See discussion in python/mypy#3465

pep-0484.txt

@@ -1590,8 +1590,9 @@ from ``UserId`` where ``int`` is expected. Examples::
         num = UserId(5) + 1     # type: int
 
 ``NewType`` accepts exactly two arguments: a name for the new unique type,
-and a base class. The latter should be a proper class, i.e.,
-not a type construct like ``Union``, etc. The function returned by ``NewType``
+and a base class. The latter should be a proper class (i.e.,
+not a type construct like ``Union``, etc.), or another unique type created
+by calling ``NewType``. The function returned by ``NewType``
 accepts only one argument; this is equivalent to supporting only one
 constructor accepting an instance of the base class (see above). Example::