This started failing once again on koobs-freebsd-current:

======================================================================
ERROR: test_chown (test.test_os.ChownFileTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.x.koobs-freebsd-current/build/Lib/test/test_os.py", line 1168, in test_chown
    os.chown(support.TESTFN, uid, gid_1)
PermissionError: [Errno 1] Operation not permitted: '@test_15221_tmp'

Looking into this a bit further:

buildbot defaults (via twisted) to a umask of 077 for the build worker build/ directory, resulting in build directories with permissions as follows:

drwx------   3 buildbot  buildbot         4 May 17 12:36 2.7.koobs-freebsd-current
drwx------   3 buildbot  buildbot         4 Aug 21 11:28 2.7.koobs-freebsd-current.nondebug

Looking inside these directories to where TESTFN's (I think) are written, we find:

[root@CURRENT-amd64:/usr/home/buildbot/python/2.7.koobs-freebsd-current/build/build] ls -la
total 18
drwx------   8 buildbot  buildbot   8 Sep 27 17:03 .
drwx------  19 buildbot  buildbot  45 Sep 27 16:59 ..
drwx------   2 buildbot  buildbot  68 Sep 27 16:59 lib.freebsd-12.0-CURRENT-amd64-2.7-pydebug
drwx------   2 buildbot  buildbot   6 Sep 27 16:59 scripts-2.7
drwx------   4 buildbot  buildbot   4 Sep 27 16:59 temp.freebsd-12.0-CURRENT-amd64-2.7-pydebug
drwx------   2 buildbot  buildbot   2 Sep 27 16:59 test_python_45339
drwx------   2 buildbot  buildbot   2 Sep 27 17:03 test_python_46278
drwx------   2 buildbot  buildbot   2 Sep 27 17:03 test_python_47336

Does this explain the "Operation not permitted" error above?

If so, the question is, why/how is this test being run if it requires an environment within which two groups can read and/or write files?

Again looking at group membership for the buildbot user on the worker host in question (koobs-freebsd-current), the user only has a single group:

# groups buildbot
buildbot