Replace Py_NotImplemented returns with the macro form Py_RETURN_NOTIMPLEMENTED.
The macro was introduced in #12724.
diff --git a/Objects/sliceobject.c b/Objects/sliceobject.c
index 53cc951..92464ab 100644
--- a/Objects/sliceobject.c
+++ b/Objects/sliceobject.c
@@ -326,10 +326,8 @@
     PyObject *t2;
     PyObject *res;
 
-    if (!PySlice_Check(v) || !PySlice_Check(w)) {
-        Py_INCREF(Py_NotImplemented);
-        return Py_NotImplemented;
-    }
+    if (!PySlice_Check(v) || !PySlice_Check(w))
+        Py_RETURN_NOTIMPLEMENTED;
 
     if (v == w) {
         /* XXX Do we really need this shortcut?