WeakSet should allow discard and remove on items that can't have weak references

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-04-19.05:20:30.848>
labels = ['3.8', 'type-feature', 'library']
title = "WeakSet should allow discard and remove on items that can't have weak references"
updated_at = <Date 2018-03-27.03:24:05.729>
user = 'https://github.com/donkopotamus'

bugs.python.org fields:

activity = <Date 2018-03-27.03:24:05.729>
actor = 'rhettinger'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-04-19.05:20:30.848>
creator = 'donkopotamus'
dependencies = []
files = []
hgrepos = []
issue_num = 30100
keywords = []
message_count = 7.0
messages = ['291861', '291867', '291868', '291878', '291880', '291918', '314495']
nosy_count = 6.0
nosy_names = ['fdrake', 'rhettinger', 'pitrou', 'stutzbach', 'serhiy.storchaka', 'donkopotamus']
pr_nums = ['1176']
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30100'
versions = ['Python 3.8']