gh-38930: expunge is_commutative from plural

    
to really fix #38047

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- [x] The title is concise and informative.
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preview.
    
URL: #38930
Reported by: Frédéric Chapoton
Reviewer(s): Martin Rubey

src/sage/rings/polynomial/plural.pyx

@@ -99,6 +99,16 @@ TESTS::
     sage: TestSuite(P).run()
     sage: loads(dumps(P)) is P
     True
+
+    sage: A.<x,y,z> = FreeAlgebra(QQ, 3)
+    sage: P = A.g_algebra(relations={y*x:-x*y}, order = 'lex')
+    sage: P.is_commutative()
+    False
+
+    sage: R.<x,y,z> = FreeAlgebra(QQ, 3)
+    sage: P = R.g_algebra(relations={}, order='lex')
+    sage: P.is_commutative()
+    True
 """
 from cysignals.memory cimport sig_malloc, sig_free
 
@@ -676,22 +686,7 @@ cdef class NCPolynomialRing_plural(Ring):
         """
         return self._term_order
 
-    def is_commutative(self):
-        """
-        Return ``False``.
-
-        .. TODO:: Provide a mathematically correct answer.
-
-        EXAMPLES::
-
-            sage: A.<x,y,z> = FreeAlgebra(QQ, 3)
-            sage: P = A.g_algebra(relations={y*x:-x*y}, order = 'lex')
-            sage: P.is_commutative()
-            False
-        """
-        return False
-
-    def is_field(self, *args, **kwargs):
+    def is_field(self, *args, **kwargs) -> bool:
         """
         Return ``False``.