The issue was in fixed in Python 3.7 with a change made for optimization, not directly to fix this issue:

changeset:   105547:b29c719d5992
tag:         tip
user:        Victor Stinner <victor.stinner@gmail.com>
date:        Fri Dec 09 14:24:02 2016 +0100
files:       Modules/_asynciomodule.c
description:
Use _PyObject_CallMethodIdObjArgs() in _asyncio

Issue #28915: Replace _PyObject_CallMethodId() with
_PyObject_CallMethodIdObjArgs() when the format string was only made of "O"
formats, PyObject* arguments.

_PyObject_CallMethodIdObjArgs() avoids the creation of a temporary tuple and
doesn't have to parse a format string.