Ensure Perspective loads in notebooks

[philippjfr]

Small oversight that led to Perspective not loading in notebooks (and notebook exports, which includes the docs).

Fixes #6612

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 82.94%. Comparing base (3079c49) to head (fbd3a62).

Files	Patch %	Lines
panel/models/perspective.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6626      +/-   ##
==========================================
- Coverage   82.95%   82.94%   -0.02%     
==========================================
  Files         313      313              
  Lines       46090    46092       +2     
==========================================
- Hits        38234    38229       -5     
- Misses       7856     7863       +7     

Flag	Coverage Δ
ui-tests	40.02% <0.00%> (-0.01%)	⬇️
unitexamples-tests	71.22% <66.66%> (-0.02%)	⬇️

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