Make the API of argparse.HelpFormatter public

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-11-24.09:53:23.152>
labels = ['3.8', 'type-feature', 'library']
title = 'Make the API of argparse.HelpFormatter public'
updated_at = <Date 2018-01-09.07:11:00.335>
user = 'https://github.com/Bernhard10'

bugs.python.org fields:

activity = <Date 2018-01-09.07:11:00.335>
actor = 'paul.j3'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-11-24.09:53:23.152>
creator = 'Bernhard10'
dependencies = []
files = []
hgrepos = []
issue_num = 32123
keywords = []
message_count = 4.0
messages = ['306879', '306926', '307761', '309687']
nosy_count = 3.0
nosy_names = ['terry.reedy', 'paul.j3', 'Bernhard10']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32123'
versions = ['Python 3.8']