Attempt to get a backtrace out of crash on MacOS

[njsmith]

No description provided.

[codecov]

Codecov Report

Merging #582 into master will decrease coverage by 3.69%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #582     +/-   ##
=========================================
- Coverage   99.27%   95.58%   -3.7%     
=========================================
  Files          89       89             
  Lines       10628    10640     +12     
  Branches      747      747             
=========================================
- Hits        10551    10170    -381     
- Misses         59      424    +365     
- Partials       18       46     +28

Impacted Files	Coverage Δ
trio/_core/_io_epoll.py	0% <0%> (-100%)	⬇️
trio/_core/_windows_cffi.py	0% <0%> (-91.31%)	⬇️
trio/_core/_io_windows.py	0% <0%> (-77.1%)	⬇️
trio/_core/tests/test_windows.py	23.33% <0%> (-76.67%)	⬇️
trio/_core/tests/test_epoll.py	27.77% <0%> (-72.23%)	⬇️
trio/_util.py	86.58% <0%> (-13.42%)	⬇️
trio/_highlevel_open_unix_stream.py	66.66% <0%> (-11.12%)	⬇️
trio/_core/tests/tutil.py	90.9% <0%> (-9.1%)	⬇️
trio/_socket.py	91.96% <0%> (-8.04%)	⬇️
trio/tests/test_highlevel_open_unix_stream.py	93.54% <0%> (-6.46%)	⬇️
... and 13 more

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[njsmith]

I think this has served its purpose... see MagicStack/immutables#7 and #583

[njsmith]

Re-opening to test immutables 0.6, as requested