Avoid refcycles in run exc

[graingert]

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 100.00000%. Comparing base (3260974) to head (d0ada54). :warning: Report is 429 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3120   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             124          124           
  Lines           18427        18433    +6     
  Branches         1215         1215           
===============================================
+ Hits            18427        18433    +6     

Files with missing lines	Coverage Δ
src/trio/_core/_run.py	100.00000% <100.00000%> (ø)

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[graingert]

I'm currently trying to work out if this change makes it possible to test https://github.com/python-trio/trio/blob/12e8395cf6939ec9f00df122487dc66ef774fec8/src/trio/_core/_run.py#L2790