Ensure type annotation allows str and PathLike objects on pn.serve

[philippjfr]

Fixes #3649

[codecov]

Codecov Report

Merging #4336 (677e8e1) into main (47f231b) will decrease coverage by 0.01%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##             main    #4336      +/-   ##
==========================================
- Coverage   82.75%   82.74%   -0.01%     
==========================================
  Files         240      240              
  Lines       34558    34562       +4     
==========================================
  Hits        28598    28598              
- Misses       5960     5964       +4     

Flag	Coverage Δ
ui-tests	38.29% <0.00%> (-0.02%)	⬇️
unitexamples-tests	73.78% <57.14%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
panel/io/server.py	74.95% <57.14%> (-0.19%)	⬇️
panel/tests/util.py	89.32% <0.00%> (-1.95%)	⬇️

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