Refactor traceback.py to make TracebackException more extensible.

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-09-09.22:41:24.200>
labels = ['type-feature', 'library', '3.11']
title = 'Refactor traceback.py to make TracebackException more extensible.'
updated_at = <Date 2021-09-09.23:05:39.751>
user = 'https://github.com/Abrosimov-a-a'

bugs.python.org fields:

activity = <Date 2021-09-09.23:05:39.751>
actor = 'abrosimov.a.a'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-09-09.22:41:24.200>
creator = 'abrosimov.a.a'
dependencies = []
files = []
hgrepos = []
issue_num = 45158
keywords = []
message_count = 2.0
messages = ['401528', '401529']
nosy_count = 1.0
nosy_names = ['abrosimov.a.a']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue45158'
versions = ['Python 3.11']