In Linuxes with ACLs enabled, the following tests fail, as Steve Dower discovered in https://mail.python.org/pipermail/python-dev/2019-March/156929.html:

======================================================================
FAIL: test_mode (test.test_os.MakedirTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/osboxes/Documents/cpython/Lib/test/test_os.py", line 1157, in test_mode
    self.assertEqual(os.stat(parent).st_mode & 0o777, 0o775)
AssertionError: 493 != 509

======================================================================
FAIL: test_open_mode (test.test_pathlib.PosixPathTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/osboxes/Documents/cpython/Lib/test/test_pathlib.py", line 2104, in test_open_mode
    self.assertEqual(stat.S_IMODE(st.st_mode), 0o666)
AssertionError: 420 != 438

======================================================================
FAIL: test_touch_mode (test.test_pathlib.PosixPathTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/osboxes/Documents/cpython/Lib/test/test_pathlib.py", line 2117, in test_touch_mode
    self.assertEqual(stat.S_IMODE(st.st_mode), 0o666)
AssertionError: 420 != 438


POSIX.1e is supported by major distros even though it's officially withdrawn; see https://en.wikipedia.org/wiki/Access_control_list#Filesystem_ACLs .