IDLE: use rlcompleter suffixed for completions

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-06-06.19:23:27.509>
labels = []
title = 'IDLE: use rlcompleter suffixed for completions'
updated_at = <Date 2020-06-06.19:23:27.509>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2020-06-06.19:23:27.509>
actor = 'terry.reedy'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2020-06-06.19:23:27.509>
creator = 'terry.reedy'
dependencies = []
files = []
hgrepos = []
issue_num = 40892
keywords = []
message_count = 1.0
messages = ['370845']
nosy_count = 1.0
nosy_names = ['terry.reedy']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue40892'
versions = []