Cannot use ProcessPoolExecutor if in a decorator?

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-08-18.17:21:46.050>
labels = ['3.7', '3.8', 'type-bug', 'library', '3.9']
title = 'Cannot use ProcessPoolExecutor if in a decorator?'
updated_at = <Date 2020-08-19.03:12:40.247>
user = 'https://bugs.python.org/bobfanglondon'

bugs.python.org fields:

activity = <Date 2020-08-19.03:12:40.247>
actor = 'aeros'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-08-18.17:21:46.050>
creator = 'bob.fang.london'
dependencies = []
files = []
hgrepos = []
issue_num = 41577
keywords = []
message_count = 2.0
messages = ['375614', '375632']
nosy_count = 4.0
nosy_names = ['pitrou', 'alexandre.vassalotti', 'aeros', 'bob.fang.london']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41577'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9']