erp5_oauth2_authorisation: fix test_expired_tokens in python2

we have a test_expired_tokens failed only in python2

Because

in python2, api_jwt.py use now = timegm(datetime.utcnow().utctimetuple())

in python3, apt_jwt.py use now = timegm(datetime.now(tz=timezone.utc).utctimetuple())

so mock utcnow as well