[multiprocessing] Calling pool.terminate() from an error_callback causes deadlock

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-01-19.01:08:51.183>
labels = ['3.7', 'type-bug', 'library']
title = '[multiprocessing] Calling pool.terminate() from an error_callback causes deadlock'
updated_at = <Date 2021-01-19.01:10:42.325>
user = 'https://github.com/sjelin'

bugs.python.org fields:

activity = <Date 2021-01-19.01:10:42.325>
actor = 'sjelin'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-01-19.01:08:51.183>
creator = 'sjelin'
dependencies = []
files = []
hgrepos = []
issue_num = 42963
keywords = []
message_count = 1.0
messages = ['385240']
nosy_count = 1.0
nosy_names = ['sjelin']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42963'
versions = ['Python 3.6', 'Python 3.7']