Make namedexpr_test a proper NamedExpr class

CHANGELOG.rst

@@ -9,6 +9,7 @@ Unreleased (XXXX-XX-XX)
 - Dropped Support for Python 2.7, 3.4, 3.5
 - It's possible to use ``pathlib.Path`` objects now in the API
 - The stubs are gone, we are now using annotations
+- ``namedexpr_test`` nodes are now a proper class called ``NamedExpr``
 - A lot of smaller refactorings
 
 0.7.1 (2020-07-24)