The Ubuntu test runner on Azure have been blocked and eventually failed by an issue related to tkinter tests. My PRs ran into the issue at least three times today.

Example: https://github.com/python/cpython/pull/23360/checks?check_run_id=1418826919

0:19:06 load avg: 0.00 running: test_ttk_guionly (19 min 1 sec)
0:19:36 load avg: 0.00 running: test_ttk_guionly (19 min 31 sec)
0:20:05 load avg: 0.00 [425/425/1] test_ttk_guionly crashed (Exit code 1)
Timeout (0:20:00)!
Thread 0x00007f7b8751d080 (most recent call first):
  File "/home/runner/work/cpython/cpython/Lib/tkinter/__init__.py", line 696 in wait_visibility
  File "/home/runner/work/cpython/cpython/Lib/tkinter/test/test_ttk/test_widgets.py", line 1539 in test_heading_callback
  File "/home/runner/work/cpython/cpython/Lib/unittest/case.py", line 549 in _callTestMethod
  File "/home/runner/work/cpython/cpython/Lib/unittest/case.py", line 592 in run
  File "/home/runner/work/cpython/cpython/Lib/unittest/case.py", line 652 in __call__
  File "/home/runner/work/cpython/cpython/Lib/unittest/suite.py", line 122 in run
  File "/home/runner/work/cpython/cpython/Lib/unittest/suite.py", line 84 in __call__
  File "/home/runner/work/cpython/cpython/Lib/unittest/suite.py", line 122 in run
  File "/home/runner/work/cpython/cpython/Lib/unittest/suite.py", line 84 in __call__
  File "/home/runner/work/cpython/cpython/Lib/unittest/runner.py", line 176 in run
  File "/home/runner/work/cpython/cpython/Lib/test/support/__init__.py", line 959 in _run_suite
  File "/home/runner/work/cpython/cpython/Lib/test/support/__init__.py", line 1082 in run_unittest
  File "/home/runner/work/cpython/cpython/Lib/test/test_ttk_guionly.py", line 31 in test_main
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/runtest.py", line 236 in _runtest_inner2
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/runtest.py", line 272 in _runtest_inner
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/runtest.py", line 142 in _runtest
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/runtest.py", line 195 in runtest
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/runtest_mp.py", line 81 in run_tests_worker
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/main.py", line 659 in _main
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/main.py", line 639 in main
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/main.py", line 717 in main
  File "/home/runner/work/cpython/cpython/Lib/test/regrtest.py", line 43 in _main
  File "/home/runner/work/cpython/cpython/Lib/test/regrtest.py", line 47 in <module>
  File "/home/runner/work/cpython/cpython/Lib/runpy.py", line 87 in _run_code
  File "/home/runner/work/cpython/cpython/Lib/runpy.py", line 197 in _run_module_as_main
...

test_heading_callback (tkinter.test.test_ttk.test_widgets.TreeviewTest) ... Timeout (0:20:00)!
Thread 0x00007f0461140080 (most recent call first):
  File "/home/runner/work/cpython/cpython/Lib/tkinter/__init__.py", line 696 in wait_visibility
  File "/home/runner/work/cpython/cpython/Lib/tkinter/test/test_ttk/test_widgets.py", line 1539 in test_heading_callback
  File "/home/runner/work/cpython/cpython/Lib/unittest/case.py", line 549 in _callTestMethod
  File "/home/runner/work/cpython/cpython/Lib/unittest/case.py", line 592 in run
  File "/home/runner/work/cpython/cpython/Lib/unittest/case.py", line 652 in __call__
  File "/home/runner/work/cpython/cpython/Lib/unittest/suite.py", line 122 in run
  File "/home/runner/work/cpython/cpython/Lib/unittest/suite.py", line 84 in __call__
  File "/home/runner/work/cpython/cpython/Lib/unittest/suite.py", line 122 in run
  File "/home/runner/work/cpython/cpython/Lib/unittest/suite.py", line 84 in __call__
  File "/home/runner/work/cpython/cpython/Lib/unittest/runner.py", line 176 in run
  File "/home/runner/work/cpython/cpython/Lib/test/support/__init__.py", line 959 in _run_suite
  File "/home/runner/work/cpython/cpython/Lib/test/support/__init__.py", line 1082 in run_unittest
  File "/home/runner/work/cpython/cpython/Lib/test/test_ttk_guionly.py", line 31 in test_main
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/runtest.py", line 236 in _runtest_inner2
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/runtest.py", line 272 in _runtest_inner
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/runtest.py", line 155 in _runtest
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/runtest.py", line 195 in runtest
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/main.py", line 319 in rerun_failed_tests
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/main.py", line 696 in _main
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/main.py", line 639 in main
  File "/home/runner/work/cpython/cpython/Lib/test/libregrtest/main.py", line 717 in main
  File "/home/runner/work/cpython/cpython/Lib/test/__main__.py", line 2 in <module>
  File "/home/runner/work/cpython/cpython/Lib/runpy.py", line 87 in _run_code
  File "/home/runner/work/cpython/cpython/Lib/runpy.py", line 197 in _run_module_as_main
Makefile:1202: recipe for target 'buildbottest' failed
make: *** [buildbottest] Error 1
Error: Process completed with exit code 2.