unittest overrides more serious warnings filter added before unittest.main()

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-10-28.20:42:28.565>
labels = ['3.8', 'type-bug', 'library', '3.9', '3.10']
title = 'unittest overrides more serious warnings filter added before unittest.main()'
updated_at = <Date 2020-10-31.21:14:17.225>
user = 'https://bugs.python.org/yileinotused'

bugs.python.org fields:

activity = <Date 2020-10-31.21:14:17.225>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-10-28.20:42:28.565>
creator = 'yilei_not_used'
dependencies = []
files = []
hgrepos = []
issue_num = 42186
keywords = []
message_count = 4.0
messages = ['379843', '379850', '380016', '380039']
nosy_count = 4.0
nosy_names = ['terry.reedy', 'ncoghlan', 'xtreak', 'yilei_not_used']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42186'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']