Standardise more behaviours for zero-argument super() __class__ and __classcell__

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-12-07.01:24:28.059>
labels = ['type-bug']
title = 'Standardise more behaviours for zero-argument super() __class__ and __classcell__'
updated_at = <Date 2016-12-07.01:36:23.643>
user = 'https://github.com/ncoghlan'

bugs.python.org fields:

activity = <Date 2016-12-07.01:36:23.643>
actor = 'ncoghlan'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2016-12-07.01:24:28.059>
creator = 'ncoghlan'
dependencies = []
files = []
hgrepos = []
issue_num = 28891
keywords = []
message_count = 2.0
messages = ['282585', '282587']
nosy_count = 2.0
nosy_names = ['arigo', 'ncoghlan']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28891'
versions = []