Soften restriction for runtime generics in PEP 484 (#120)

Fixes python/typing#303.

See also python/mypy#2302 (which removes the restriction from mypy).

As a motivation, in Python one always can substitute expressions, so that if ``IntNode = Node[int]; IntNode()`` works, then it is reasonable to also allow ``Node[int]``, but say that the first way is preferred.

Author: ilevkivskyi

pep-0484.txt

@@ -600,12 +600,10 @@ the type (class) of the objects created by instantiating them doesn't
 record the distinction.  This behavior is called "type erasure"; it is
 common practice in languages with generics (e.g. Java, TypeScript).
 
-You cannot use the subscripted class (e.g. ``Node[int]``) directly in
-an expression -- you must define a type alias.  (This restriction
-exists because creating the subscripted class, e.g. ``Node[int]``, is
-an expensive operation -- usually many times as expensive as
-constructing an instance of it.  Using a type alias is also more
-readable.)
+It is not recommended to use the subscripted class (e.g. ``Node[int]``)
+directly in an expression -- using a type alias instead is preferred.
+(First, creating the subscripted class, e.g. ``Node[int]``, has a runtime
+cost. Second, using a type alias is more readable.)
 
 
 Arbitrary generic types as base classes