list_inplace_concat() is now expressed in terms of list_extend() which

avoids creating an intermediate tuple for iterable arguments other than
lists or tuples.

In other words, a+=b no longer requires extra memory when b is not a
list or tuple.  The list and tuple cases are unchanged.

Objects/listobject.c

@@ -707,20 +707,6 @@ listextend_internal(PyListObject *self, PyObject *b)
 	return 0;
 }
 
-static PyObject *
-list_inplace_concat(PyListObject *self, PyObject *other)
-{
-	other = PySequence_Fast(other, "argument to += must be iterable");
-	if (!other)
-		return NULL;
-
-	if (listextend_internal(self, other) < 0)
-		return NULL;
-
-	Py_INCREF(self);
-	return (PyObject *)self;
-}
-
 static PyObject *
 listextend(PyListObject *self, PyObject *b)
 {
@@ -790,6 +776,19 @@ listextend(PyListObject *self, PyObject *b)
 	return NULL;
 }
 
+static PyObject *
+list_inplace_concat(PyListObject *self, PyObject *other)
+{
+	PyObject *result;
+
+	result = listextend(self, other);
+	if (result == NULL)
+		return result;
+	Py_DECREF(result);
+	Py_INCREF(self);
+	return (PyObject *)self;
+}
+
 static PyObject *
 listpop(PyListObject *self, PyObject *args)
 {