bpo-43149: Improve error message for exception group without parentheses (GH-24467)

diff --git a/Lib/test/test_syntax.py b/Lib/test/test_syntax.py
index 70dd22c..47df057 100644
--- a/Lib/test/test_syntax.py
+++ b/Lib/test/test_syntax.py
@@ -835,6 +835,39 @@
      ...
    SyntaxError: invalid syntax
 
+Check that an exception group with missing parentheses
+raise a custom exception
+
+   >>> try:
+   ...   pass
+   ... except A, B:
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: exception group must be parenthesized
+
+   >>> try:
+   ...   pass
+   ... except A, B, C:
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: exception group must be parenthesized
+
+   >>> try:
+   ...   pass
+   ... except A, B, C as blech:
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: exception group must be parenthesized
+
+   >>> try:
+   ...   pass
+   ... except A, B, C as blech:
+   ...   pass
+   ... finally:
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: exception group must be parenthesized
+
 
 >>> f(a=23, a=234)
 Traceback (most recent call last):