Adding the "with" statement support to ContextVar

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-06-08.02:38:29.223>
labels = ['type-feature', 'library', '3.11']
title = 'Adding the "with" statement support to ContextVar'
updated_at = <Date 2021-06-09.08:46:39.108>
user = 'https://github.com/achimnol'

bugs.python.org fields:

activity = <Date 2021-06-09.08:46:39.108>
actor = 'achimnol'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-06-08.02:38:29.223>
creator = 'achimnol'
dependencies = []
files = []
hgrepos = []
issue_num = 44343
keywords = []
message_count = 2.0
messages = ['395302', '395397']
nosy_count = 5.0
nosy_names = ['njs', 'yselivanov', 'corona10', 'achimnol', 'aeros']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue44343'
versions = ['Python 3.11']