Extra `DeprecationWarning` when running `lib2to3` tests

[sobolevn]

BPO	46815
Nosy	@benjaminp, @ambv, @JelleZijlstra, @sobolevn
PRs	gh-90971: suppress deprecation warning in test_lib2to3 #31464

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-02-21.10:22:11.314>
labels = ['type-bug', 'tests', '3.9', '3.10', '3.11']
title = 'Extra `DeprecationWarning` when running `lib2to3` tests'
updated_at = <Date 2022-02-21.10:23:36.136>
user = 'https://github.com/sobolevn'

bugs.python.org fields:

activity = <Date 2022-02-21.10:23:36.136>
actor = 'sobolevn'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Tests']
creation = <Date 2022-02-21.10:22:11.314>
creator = 'sobolevn'
dependencies = []
files = []
hgrepos = []
issue_num = 46815
keywords = ['patch']
message_count = 1.0
messages = ['413643']
nosy_count = 4.0
nosy_names = ['benjamin.peterson', 'lukasz.langa', 'JelleZijlstra', 'sobolevn']
pr_nums = ['31464']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46815'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[sobolevn]

I first noticed it in the buildbot logs:

0:24:42 load avg: 3.87 [430/431/1] test_lib2to3 passed (1 min 38 sec)
<string>:2: DeprecationWarning: lib2to3 package is deprecated and may not be able to parse Python 3.10+

But, it also happens locally:

» ./python.exe Lib/test/test_lib2to3.py
/Users/sobolev/Desktop/cpython/Lib/unittest/loader.py:350: DeprecationWarning: lib2to3 package is deprecated and may not be able to parse Python 3.10+
  __import__(name)
Refactor file: /Users/sobolev/Desktop/cpython/Lib/lib2to3/refactor.py

After my patch it is gone:

» ./python.exe Lib/test/test_lib2to3.py
Refactor file: /Users/sobolev/Desktop/cpython/Lib/lib2to3/refactor.py