I don't know if this is a buildbot, test or 3.9-specific issue but this commit appears to have introduced a permanent initial failure (but success on retry) in test_pickle on both Windows 10 3.9 builders.  First failure for my builder at https://buildbot.python.org/all/#/builders/425/builds/450

Fatal Python error: _Py_CheckRecursiveCall: Cannot recover from stack overflow.
0:14:47 load avg: 4.57 Re-running failed tests in verbose mode
0:14:47 load avg: 4.57 Re-running test_pickle in verbose mode

The 3.x and 3.10 builders seem fine, and the second try on 3.9 always seems to succeed (perhaps because it's just a single test running at that point), so this is only being reported as a buildbot warning.