Add a "message" action to warnings, to trigger for every *unique* message

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-08-26.09:11:26.612>
labels = ['type-feature', 'library', '3.9']
title = 'Add a "message" action to warnings, to trigger for every *unique* message'
updated_at = <Date 2020-08-26.09:11:26.612>
user = 'https://github.com/masklinn'

bugs.python.org fields:

activity = <Date 2020-08-26.09:11:26.612>
actor = 'xmorel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-08-26.09:11:26.612>
creator = 'xmorel'
dependencies = []
files = []
hgrepos = []
issue_num = 41641
keywords = []
message_count = 1.0
messages = ['375924']
nosy_count = 1.0
nosy_names = ['xmorel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41641'
versions = ['Python 3.9']