unittest blocks when testing function using multiprocessing.Pool with state spawn

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-03-12.13:12:55.379>
labels = ['type-bug']
title = 'unittest blocks when testing function using multiprocessing.Pool with state spawn'
updated_at = <Date 2018-03-13.22:06:33.152>
user = 'https://bugs.python.org/KennethChik'

bugs.python.org fields:

activity = <Date 2018-03-13.22:06:33.152>
actor = 'pitrou'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2018-03-12.13:12:55.379>
creator = 'Kenneth Chik'
dependencies = []
files = []
hgrepos = []
issue_num = 33054
keywords = []
message_count = 2.0
messages = ['313648', '313787']
nosy_count = 2.0
nosy_names = ['pitrou', 'Kenneth Chik']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33054'
versions = ['Python 3.6']