I can reproduce this by running test_lib2to3 twice or running test___all__ with test_lib2to3

./python -m test test_lib2to3 test_lib2to3
0:00:00 load avg: 0.03 Run tests sequentially
0:00:00 load avg: 0.03 [1/2] test_lib2to3
0:00:13 load avg: 0.25 [2/2] test_lib2to3
test test_lib2to3 crashed -- Traceback (most recent call last):
  File "/root/cpython/Lib/test/libregrtest/runtest.py", line 272, in _runtest_inner
    refleak = _runtest_inner2(ns, test_name)
  File "/root/cpython/Lib/test/libregrtest/runtest.py", line 223, in _runtest_inner2
    the_module = importlib.import_module(abstest)
  File "/root/cpython/Lib/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 790, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/root/cpython/Lib/test/test_lib2to3.py", line 5, in <module>
    from lib2to3.tests import load_tests
  File "/root/cpython/Lib/contextlib.py", line 124, in __exit__
    next(self.gen)
  File "/root/cpython/Lib/test/support/warnings_helper.py", line 179, in _filterwarnings
    raise AssertionError("filter (%r, %s) did not catch any warning" %
AssertionError: filter ('', PendingDeprecationWarning) did not catch any warning

test_lib2to3 failed

== Tests result: FAILURE ==

1 test OK.

1 test failed:
    test_lib2to3

Total duration: 13.5 sec
Tests result: FAILURE

./python -m test test___all__ test_lib2to3     
0:00:00 load avg: 0.05 Run tests sequentially
0:00:00 load avg: 0.05 [1/2] test___all__
0:00:00 load avg: 0.05 [2/2] test_lib2to3
test test_lib2to3 crashed -- Traceback (most recent call last):
  File "/root/cpython/Lib/test/libregrtest/runtest.py", line 272, in _runtest_inner
    refleak = _runtest_inner2(ns, test_name)
  File "/root/cpython/Lib/test/libregrtest/runtest.py", line 223, in _runtest_inner2
    the_module = importlib.import_module(abstest)
  File "/root/cpython/Lib/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 790, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/root/cpython/Lib/test/test_lib2to3.py", line 5, in <module>
    from lib2to3.tests import load_tests
  File "/root/cpython/Lib/contextlib.py", line 124, in __exit__
    next(self.gen)
  File "/root/cpython/Lib/test/support/warnings_helper.py", line 179, in _filterwarnings
    raise AssertionError("filter (%r, %s) did not catch any warning" %
AssertionError: filter ('', PendingDeprecationWarning) did not catch any warning

test_lib2to3 failed

== Tests result: FAILURE ==

1 test OK.

1 test failed:
    test_lib2to3

Total duration: 1.1 sec
Tests result: FAILURE