Test syntax error on comma-less tuple-style sequence patterns (#115485)

Adds a test that length-1 tuple-style sequence patterns must end in a comma, since there isn't currently one.

Spotted while reviewing Cython's proposed implementation of the pattern matching syntax (https://github.com/cython/cython/pull/4897#discussion_r1489177169) where there was a bug my the reimplementation that wasn't caught against the CPython tests here.

Lib/test/test_patma.py

@@ -2957,6 +2957,14 @@ class TestSyntaxErrors(unittest.TestCase):
                 pass
         """)
 
+    def test_len1_tuple_sequence_pattern_comma(self):
+        # correct syntax would be `case(*x,):`
+        self.assert_syntax_error("""
+        match ...:
+            case (*x):
+                pass
+        """)
+
     def test_mapping_pattern_keys_may_only_match_literals_and_attribute_lookups(self):
         self.assert_syntax_error("""
         match ...: