Ok, got it. I think __ipow__ should be a ternaryfunc, like so:

diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 403f3caaee..914d076b5c 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -7032,7 +7032,7 @@ static slotdef slotdefs[] = {
     IBSLOT("__imod__", nb_inplace_remainder, slot_nb_inplace_remainder,
            wrap_binaryfunc, "%="),
     IBSLOT("__ipow__", nb_inplace_power, slot_nb_inplace_power,
-           wrap_binaryfunc, "**="),
+           wrap_ternaryfunc, "**="),
     IBSLOT("__ilshift__", nb_inplace_lshift, slot_nb_inplace_lshift,
            wrap_binaryfunc, "<<="),
     IBSLOT("__irshift__", nb_inplace_rshift, slot_nb_inplace_rshift,



On the other hand it is odd if "**=" can never use the third argument.