__reduce__ API specs for objects with __slots__

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-09-18.13:50:59.485>
labels = ['3.8', 'type-bug', '3.7', '3.9', 'docs']
title = '__reduce__ API specs for objects with __slots__'
updated_at = <Date 2019-09-18.13:50:59.485>
user = 'https://github.com/crusaderky'

bugs.python.org fields:

activity = <Date 2019-09-18.13:50:59.485>
actor = 'crusaderky'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2019-09-18.13:50:59.485>
creator = 'crusaderky'
dependencies = []
files = []
hgrepos = []
issue_num = 38214
keywords = []
message_count = 1.0
messages = ['352728']
nosy_count = 2.0
nosy_names = ['docs@python', 'crusaderky']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38214'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']