bpo-40334: Spacialized error message for invalid args after bare '*' (GH-19865)

When parsing things like `def f(*): pass` the old parser used to output `SyntaxError: named arguments must follow bare *`, which the new parser wasn't able to do.
diff --git a/Lib/test/test_syntax.py b/Lib/test/test_syntax.py
index e7468ca..0c0fc48 100644
--- a/Lib/test/test_syntax.py
+++ b/Lib/test/test_syntax.py
@@ -627,9 +627,9 @@
     Traceback (most recent call last):
     SyntaxError: cannot assign to __debug__
 
-    # >>> with (lambda *:0): pass
-    # Traceback (most recent call last):
-    # SyntaxError: named arguments must follow bare *
+    >>> with (lambda *:0): pass
+    Traceback (most recent call last):
+    SyntaxError: named arguments must follow bare *
 
 Corner-cases that used to crash: