Add roadmap.txt section to idlelib

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2017-05-21.09:24:22.859>
labels = ['expert-IDLE', 'type-feature', '3.10']
title = 'Add roadmap.txt section to idlelib'
updated_at = <Date 2020-06-07.23:02:33.300>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2020-06-07.23:02:33.300>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2017-05-21.09:24:22.859>
creator = 'terry.reedy'
dependencies = []
files = ['46979']
hgrepos = []
issue_num = 30422
keywords = []
message_count = 15.0
messages = ['294089', '294093', '294118', '296513', '296809', '297092', '297140', '297146', '297149', '297173', '297181', '297184', '297187', '297248', '297251']
nosy_count = 4.0
nosy_names = ['terry.reedy', 'louielu', 'cheryl.sabella', '18z']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30422'
versions = ['Python 3.10']