argparse.ArgumentDefaultsHelpFormatter sometimes provides inaccurate documentation of defaults, so they should be overrideable

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-11-19.00:17:09.209>
labels = ['3.7', 'type-feature', 'library']
title = 'argparse.ArgumentDefaultsHelpFormatter sometimes provides inaccurate documentation of defaults, so they should be overrideable'
updated_at = <Date 2021-06-03.18:03:15.796>
user = 'https://github.com/pde'

bugs.python.org fields:

activity = <Date 2021-06-03.18:03:15.796>
actor = 'wim.glenn'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-11-19.00:17:09.209>
creator = 'pde'
dependencies = []
files = []
hgrepos = []
issue_num = 28742
keywords = []
message_count = 7.0
messages = ['281183', '281184', '281189', '281590', '282102', '282507', '341608']
nosy_count = 4.0
nosy_names = ['paul.j3', 'wim.glenn', 'pde', 'anthonypjshaw']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue28742'
versions = ['Python 3.7']