concurrent.futures.threading: Add a timeout to Executor shutdown() method

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-09-06.18:49:39.355>
labels = ['3.8', 'type-feature', 'library']
title = 'concurrent.futures.threading: Add a timeout to Executor shutdown() method'
updated_at = <Date 2021-09-26.21:19:09.922>
user = 'https://bugs.python.org/PatrikDufresne'

bugs.python.org fields:

activity = <Date 2021-09-26.21:19:09.922>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-09-06.18:49:39.355>
creator = 'Patrik Dufresne'
dependencies = []
files = []
hgrepos = []
issue_num = 27978
keywords = []
message_count = 1.0
messages = ['274599']
nosy_count = 2.0
nosy_names = ['pitrou', 'Patrik Dufresne']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue27978'
versions = ['Python 3.8']