style(tree): format with black

[TomJGooding]

Run black over recent changes to tree.py. This is currently causing PR checks to fail.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.23%. Comparing base (9b4e3f1) to head (310defc).
Report is 21 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3421      +/-   ##
==========================================
+ Coverage   95.18%   98.23%   +3.05%     
==========================================
  Files          74       74              
  Lines        8059     8060       +1     
==========================================
+ Hits         7671     7918     +247     
+ Misses        388      142     -246     

Flag	Coverage Δ
unittests	98.23% <ø> (+3.05%)	⬆️

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[TomJGooding]

Sorry @willmcgugan I know you have a backlog, but just a gentle ping as merging this now might save you time later.