singledispatchmethod should expose registry of all known overloads

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-01-16.21:46:19.079>
labels = ['type-feature', 'library', '3.11']
title = 'singledispatchmethod should expose registry of all known overloads'
updated_at = <Date 2021-12-09.15:14:16.576>
user = 'https://github.com/Kentzo'

bugs.python.org fields:

activity = <Date 2021-12-09.15:14:16.576>
actor = 'hongweipeng'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-01-16.21:46:19.079>
creator = 'Ilya.Kulakov'
dependencies = []
files = []
hgrepos = []
issue_num = 42943
keywords = ['patch']
message_count = 1.0
messages = ['385151']
nosy_count = 6.0
nosy_names = ['ncoghlan', 'methane', 'lukasz.langa', 'Ilya.Kulakov', 'hongweipeng', 'AlexWaygood']
pr_nums = ['30007']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue42943'
versions = ['Python 3.11']