@vstinner vstinner removed the needs backport to 3.7 label just now

I did that to prevent merging the change too early. First we have to make sure that the test pass on the Gentoo buildbot.

@terryjreedy, this is a great fix! I just added a few minor comments.

@vstinner Removing the backport label does not prevent merging. Omitting both news and skip news, which I did intentionally, with the intention to add one later, did prevent merging, at least initially. Adding skip news turned the button green and enabled merging ;-).

Now, how does one force a particular buildbot to test a particular PR without merging?
I see a [Force] button but no place to enter a PR #.

@vstinner Removing the backport label does not prevent merging.

I don't want to block this PR. I want to block a backport to 3.7 until we can double check that this change fix the Gentoo buildbot on master.

Now, how does one force a particular buildbot to test a particular PR without merging?

Use https://devguide.python.org/buildbots/#custom-builders to test your PR on all buildbots at once.

If you only want to test on one specific buildbot, contact the buildbot worker owner who may or may not answer :-)

I've triggered a run on all buildbots via custom-builders.

EDIT: One of the relevant Gentoo builders had an error running the tests for the custom build:

Upon execvpe b'./target/bin/pythoncustom' [b'./target/bin/pythoncustom', b'-Wdefault', b'-bb', b'-E', b'-m', b'test.regrtest', b'-rwW', b'-uall', b'-j2'] in environment id 3054377756
:Traceback (most recent call last):
  File "/buildbot/venv/lib/python3.6/site-packages/twisted/internet/process.py", line 445, in _fork
    environment)
  File "/buildbot/venv/lib/python3.6/site-packages/twisted/internet/process.py", line 523, in _execChild
    os.execvpe(executable, args, environment)
  File "/usr/lib/python3.6/os.py", line 568, in execvpe
    _execvpe(file, args, env)
  File "/usr/lib/python3.6/os.py", line 583, in _execvpe
    exec_func(file, *argrest)
FileNotFoundError: [Errno 2] No such file or directory: b'./target/bin/pythoncustom'

That's annoying. Looking at https://buildbot.python.org/all/#/grid, the only normally green buildbots to turn red are the 3 Gentoo bots. The other non-refleak bot had the same error on test startup. The refleaks bot did run the test, failing with '6 not greater than 6'.
https://buildbot.python.org/all/#/builders/15/builds/12/steps/4/logs/stdio

I strongly suspect from the latter that the fix would not work on the normal Gentoo bots and that there is something peculiar about tk on the Gentoo machine. I am inclined to let this sit a bit.

In the latter, I noticed some new 'Theme Changed' messages from recent tests, including squeezer. I will look into at previous issues where I dealt with them.

I've got another idea how to make this test work properly; see PR #11585.

I merged the other solution, so closing this.