Guard is_file check when resolving resources on Windows

[philippjfr]

Fixes #4982

[codecov]

Codecov Report

Merging #4985 (500cb0f) into main (cf97774) will decrease coverage by 0.01%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##             main    #4985      +/-   ##
==========================================
- Coverage   73.45%   73.44%   -0.01%     
==========================================
  Files         271      271              
  Lines       38289    38293       +4     
==========================================
+ Hits        28124    28125       +1     
- Misses      10165    10168       +3     

Flag	Coverage Δ
unitexamples-tests	73.44% <60.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
panel/io/resources.py	87.23% <60.00%> (-0.32%)	⬇️

... and 1 file with indirect coverage changes

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