Exclude bad rdkit version for mypy check

[IAlibay]

There's an issue with rdkit 2026.03.3 when it comes to type checking - I believe it's being addressed upstream.

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 93.62%. Comparing base (8783f8d) to head (d92a3ae).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5405   +/-   ##
========================================
  Coverage    93.62%   93.62%           
========================================
  Files          182      182           
  Lines        22509    22509           
  Branches      3202     3202           
========================================
+ Hits         21073    21075    +2     
+ Misses         967      965    -2     
  Partials       469      469           

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[orbeckst]

neat work-around