I fixed this in https://github.com/python/cpython/pull/21694 . Somehow this seems to emit warning over full test run and not in individual case :( Is there a command to reproduce this always? I ran the command but it's successful for me and doesn't show the warning.

./python -Werror -m test -v test_lib2to3 -m test_load_grammar_from_subprocess
== CPython 3.10.0a7+ (heads/master:d9151cb453, Apr 13 2021, 03:10:47) [GCC 7.5.0]
== Linux-4.15.0-99-generic-x86_64-with-glibc2.27 little-endian
== cwd: /root/cpython/build/test_python_20462æ
== CPU count: 1
== encodings: locale=UTF-8, FS=utf-8
0:00:00 load avg: 0.02 Run tests sequentially
0:00:00 load avg: 0.02 [1/1] test_lib2to3
test_load_grammar_from_subprocess (lib2to3.tests.test_parser.TestPgen2Caching) ... ok

----------------------------------------------------------------------

Ran 1 test in 0.164s

OK

== Tests result: SUCCESS ==

1 test OK.

Total duration: 303 ms
Tests result: SUCCESS