IDLE: Consistently handle non .py source files

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2022-02-14.03:33:18.063>
labels = ['expert-IDLE', 'type-feature', '3.11']
title = 'IDLE: Consistently handle non .py source files'
updated_at = <Date 2022-02-14.03:33:18.063>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2022-02-14.03:33:18.063>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2022-02-14.03:33:18.063>
creator = 'terry.reedy'
dependencies = []
files = []
hgrepos = []
issue_num = 46746
keywords = []
message_count = 1.0
messages = ['413210']
nosy_count = 1.0
nosy_names = ['terry.reedy']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue46746'
versions = ['Python 3.11']