Fix black linting issue in TPR test

[IAlibay]

Just the one tiny thing was tripping up CI.

LLM / AI generated code disclosure

LLMs or other AI-powered tools (beyond simple IDE use cases) were used in this contribution: no

PR Checklist

• Issue raised/referenced?
• Tests updated/added?
• Documentation updated/added?
• package/CHANGELOG file updated?
• Is your name in package/AUTHORS? (If it is not, add it!)
• LLM/AI disclosure was updated.

Developers Certificate of Origin

I certify that I can submit this code contribution as described in the Developer Certificate of Origin, under the MDAnalysis LICENSE.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.63%. Comparing base (8783f8d) to head (ac0e73f).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5406      +/-   ##
===========================================
+ Coverage    93.62%   93.63%   +0.01%     
===========================================
  Files          182      182              
  Lines        22509    22509              
  Branches      3202     3202              
===========================================
+ Hits         21073    21076       +3     
+ Misses         967      965       -2     
+ Partials       469      468       -1     

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