Fix bug in is_viewable_param

[MarcSkovMadsen]

I was trying to use param.ClassSelector(class_=(list,tuple) with the GraphicWalker react component and I got

TypeError: issubclass() arg 1 must be a class

panel/viewable.py:1215: TypeError

This PR should add a test for this and fix the issue.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.29%. Comparing base (b43a52a) to head (c407425).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7454      +/-   ##
==========================================
+ Coverage   82.27%   82.29%   +0.02%     
==========================================
  Files         338      338              
  Lines       50772    50819      +47     
==========================================
+ Hits        41774    41824      +50     
+ Misses       8998     8995       -3     

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