Interpreter exit blocks waiting for ThreadPoolExecutor.map

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-11-19.15:02:33.692>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = 'Interpreter exit blocks waiting for ThreadPoolExecutor.map'
updated_at = <Date 2020-11-19.15:13:12.214>
user = 'https://bugs.python.org/gsakkis'

bugs.python.org fields:

activity = <Date 2020-11-19.15:13:12.214>
actor = 'stefosgiwrgos'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-11-19.15:02:33.692>
creator = 'gsakkis'
dependencies = []
files = ['49608']
hgrepos = []
issue_num = 42409
keywords = []
message_count = 1.0
messages = ['381433']
nosy_count = 2.0
nosy_names = ['gsakkis', 'stefosgiwrgos']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42409'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']