Use new _at_fork_reinit() lock method in multiprocessing

BPO	40221
Nosy	@vstinner, @corona10, @shihai1991
PRs	bpo-40221: Update ForkAwareThreadLock to use _at_fork_reinit #19477 bpo-40221: Update multiprocessing to use _at_fork_reinit #19511

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-04-14.16:38:40.314>
created_at = <Date 2020-04-07.23:03:53.125>
labels = ['type-feature', 'library', '3.9']
title = 'Use new _at_fork_reinit() lock method in multiprocessing'
updated_at = <Date 2020-04-14.16:38:40.313>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2020-04-14.16:38:40.313>
actor = 'corona10'
assignee = 'none'
closed = True
closed_date = <Date 2020-04-14.16:38:40.314>
closer = 'corona10'
components = ['Library (Lib)']
creation = <Date 2020-04-07.23:03:53.125>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 40221
keywords = ['patch']
message_count = 6.0
messages = ['365957', '366205', '366207', '366386', '366399', '366400']
nosy_count = 3.0
nosy_names = ['vstinner', 'corona10', 'shihai1991']
pr_nums = ['19477', '19511']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40221'
versions = ['Python 3.9']