Running Setuptools' test suite (which uses xdist) with PYTHONWARNDEFAULTENCODING=1, I see these warnings in the startup:
/Users/jaraco/code/pypa/setuptools/.tox/py/lib/python3.11/site-packages/execnet/gateway_base.py:163: EncodingWarning: 'encoding' argument not specified
return self._fdopen(fd, mode, bufsize)