Readd __index__ support on ipaddress objects

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-11-21.17:40:40.519>
labels = ['3.7']
title = 'Readd __index__ support on ipaddress objects'
updated_at = <Date 2017-03-22.02:01:09.175>
user = 'https://github.com/MojoVampire'

bugs.python.org fields:

activity = <Date 2017-03-22.02:01:09.175>
actor = 'josh.r'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2016-11-21.17:40:40.519>
creator = 'josh.r'
dependencies = []
files = []
hgrepos = []
issue_num = 28767
keywords = []
message_count = 3.0
messages = ['281384', '281385', '289959']
nosy_count = 4.0
nosy_names = ['ncoghlan', 'benjamin.peterson', 'serhiy.storchaka', 'josh.r']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue28767'
versions = ['Python 3.4', 'Python 3.5', 'Python 3.6', 'Python 3.7']