Fix crash when used with Raven.

[miracle2k]

See #469.

[njsmith]

Looks like you need to run yapf.

Can you throw in an entry in newsfragments/ too?

[miracle2k]

CI pass!

[codecov]

Codecov Report

Merging #599 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #599      +/-   ##
==========================================
- Coverage   99.31%   99.31%   -0.01%     
==========================================
  Files          91       91              
  Lines       10823    10822       -1     
  Branches      753      753              
==========================================
- Hits        10749    10748       -1     
  Misses         56       56              
  Partials       18       18

Impacted Files	Coverage Δ
trio/_core/_ki.py	100% <100%> (ø)	⬆️

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