bpo-39176: Improve error message for 'named assignment' (GH-17777)

diff --git a/Lib/test/test_syntax.py b/Lib/test/test_syntax.py
index 3829746..128c4da 100644
--- a/Lib/test/test_syntax.py
+++ b/Lib/test/test_syntax.py
@@ -45,7 +45,7 @@
 
 >>> (True := 1)
 Traceback (most recent call last):
-SyntaxError: cannot use named assignment with True
+SyntaxError: cannot use assignment expressions with True
 
 >>> obj.__debug__ = 1
 Traceback (most recent call last):