Make `weakref.WeakKeyDictionary.__repr__` meaningful

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-09-26.16:28:49.366>
labels = ['3.7', 'type-feature', 'library']
title = 'Make `weakref.WeakKeyDictionary.__repr__` meaningful'
updated_at = <Date 2016-09-29.17:13:51.363>
user = 'https://github.com/cool-RR'

bugs.python.org fields:

activity = <Date 2016-09-29.17:13:51.363>
actor = 'fdrake'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-09-26.16:28:49.366>
creator = 'cool-RR'
dependencies = []
files = []
hgrepos = []
issue_num = 28278
keywords = []
message_count = 3.0
messages = ['277431', '277442', '277709']
nosy_count = 4.0
nosy_names = ['fdrake', 'pitrou', 'cool-RR', 'josh.r']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue28278'
versions = ['Python 3.7']