comp(pandas): Remove observed FutureWarning

[hoxbro]

Fixes #6305

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.49%. Comparing base (eb79ab5) to head (b0258c5).
Report is 3 commits behind head on main.

Files	Patch %	Lines
holoviews/core/data/pandas.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6306       +/-   ##
===========================================
+ Coverage   17.71%   88.49%   +70.78%     
===========================================
  Files         323      323               
  Lines       68009    68047       +38     
===========================================
+ Hits        12045    60218    +48173     
+ Misses      55964     7829    -48135     

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