Add thread timeout for loop.shutdown_default_executor

[aeros]

BPO	38267
Nosy	@asvetlov, @1st1, @aeros
PRs	bpo-38267: Add thread timeout parameter to loop.shutdown_default_executor() #16360

^{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 = None
created_at = <Date 2019-09-24.19:31:44.204>
labels = ['3.9']
title = 'Add thread timeout for loop.shutdown_default_executor'
updated_at = <Date 2019-09-24.20:23:27.554>
user = 'https://github.com/aeros'

bugs.python.org fields:

activity = <Date 2019-09-24.20:23:27.554>
actor = 'aeros'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2019-09-24.19:31:44.204>
creator = 'aeros'
dependencies = []
files = []
hgrepos = []
issue_num = 38267
keywords = ['patch']
message_count = 1.0
messages = ['353115']
nosy_count = 3.0
nosy_names = ['asvetlov', 'yselivanov', 'aeros']
pr_nums = ['16360']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue38267'
versions = ['Python 3.9']

[aeros]

Currently, for the recently added coroutine loop.shutdown_default_executor(), the executor shutdown can wait indefinitely for the threads to join. Under normal circumstances, waiting on the threads is appropriate, but there should be a timeout duration in the situation that the threads unable to finish joining.

The motivation for this was based on the comments from Andrew Svetlov and Yury Selivanov in #60488.

The original idea from Andrew was to add the timeout duration as a default for a new parameter in asyncio.run() and loop.shutdown_default_executor(). However, Yury would prefer for this to be defined as a constant instead and not as a parameter for asyncio.run() to avoid the creation of an excessive number of parameters to tweak for the user.

I will attach a PR that adds the constant and the parameter for loop.shutdown_default_executor(), which will passed as an argument to thread.join().