Modernize several tests in test_importlib

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-08-20.02:48:16.674>
labels = ['type-feature', 'tests', '3.9']
title = 'Modernize several tests in test_importlib'
updated_at = <Date 2019-08-22.23:12:26.340>
user = 'https://github.com/aeros'

bugs.python.org fields:

activity = <Date 2019-08-22.23:12:26.340>
actor = 'aeros'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Tests']
creation = <Date 2019-08-20.02:48:16.674>
creator = 'aeros'
dependencies = []
files = []
hgrepos = []
issue_num = 37890
keywords = []
message_count = 10.0
messages = ['349984', '349985', '349993', '350029', '350040', '350042', '350096', '350099', '350217', '350239']
nosy_count = 5.0
nosy_names = ['brett.cannon', 'ncoghlan', 'eric.snow', 'serhiy.storchaka', 'aeros']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue37890'
versions = ['Python 3.9']