IDLE tests: don't modify tkinter.messagebox.

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2017-08-27.19:46:56.985>
labels = ['expert-IDLE', 'type-feature', '3.7']
title = "IDLE tests: don't modify tkinter.messagebox."
updated_at = <Date 2017-09-21.17:05:28.180>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2017-09-21.17:05:28.180>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2017-08-27.19:46:56.985>
creator = 'terry.reedy'
dependencies = []
files = []
hgrepos = []
issue_num = 31288
keywords = []
message_count = 2.0
messages = ['300923', '302714']
nosy_count = 1.0
nosy_names = ['terry.reedy']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31288'
versions = ['Python 3.6', 'Python 3.7']