Contextvars: Optional callbacks on state change

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-02-17.05:44:47.828>
labels = ['3.9']
title = 'Contextvars: Optional callbacks on state change'
updated_at = <Date 2020-02-20.23:51:07.428>
user = 'https://github.com/leezu'

bugs.python.org fields:

activity = <Date 2020-02-20.23:51:07.428>
actor = 'yselivanov'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2020-02-17.05:44:47.828>
creator = 'leezu'
dependencies = []
files = []
hgrepos = []
issue_num = 39660
keywords = []
message_count = 6.0
messages = ['362118', '362301', '362356', '362359', '362368', '362370']
nosy_count = 3.0
nosy_names = ['rhettinger', 'yselivanov', 'leezu']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue39660'
versions = ['Python 3.9']