pn.viewable.Viewer can now be used with pn.serve

[hoxbro]

Resolves #2768

I don't know if there should be a more advanced unit test involving calling pn.serve.

[codecov]

Codecov Report

Merging #2769 (3642463) into master (33894cd) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2769      +/-   ##
==========================================
+ Coverage   82.73%   82.74%   +0.01%     
==========================================
  Files         188      188              
  Lines       24102    24112      +10     
==========================================
+ Hits        19941    19952      +11     
+ Misses       4161     4160       -1     

Impacted Files	Coverage Δ
panel/pane/base.py	88.88% <100.00%> (+0.54%)	⬆️
panel/tests/test_viewable.py	100.00% <100.00%> (ø)

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