Issue #28727: Optimize pattern_richcompare() for a==a

A pattern is equal to itself.

Lib/test/test_re.py

@@ -1781,6 +1781,10 @@ SUBPATTERN None 0 0
     def test_pattern_compare(self):
         pattern1 = re.compile('abc', re.IGNORECASE)
 
+        # equal to itself
+        self.assertEqual(pattern1, pattern1)
+        self.assertFalse(pattern1 != pattern1)
+
         # equal
         re.purge()
         pattern2 = re.compile('abc', re.IGNORECASE)

Modules/_sre.c

@@ -2683,6 +2683,12 @@ pattern_richcompare(PyObject *lefto, PyObject *righto, int op)
     if (Py_TYPE(lefto) != &Pattern_Type || Py_TYPE(righto) != &Pattern_Type) {
         Py_RETURN_NOTIMPLEMENTED;
     }
+
+    if (lefto == righto) {
+        /* a pattern is equal to itself */
+        return PyBool_FromLong(op == Py_EQ);
+    }
+
     left = (PatternObject *)lefto;
     right = (PatternObject *)righto;