Ensure Image types can resolve pathlib objects

[philippjfr]

Fixes #2620

[codecov]

Codecov Report

Merging #3294 (b078c85) into master (51b8950) will increase coverage by 0.00%.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master    #3294   +/-   ##
=======================================
  Coverage   83.34%   83.34%           
=======================================
  Files         195      195           
  Lines       26373    26375    +2     
=======================================
+ Hits        21981    21983    +2     
  Misses       4392     4392           

Impacted Files	Coverage Δ
panel/pane/image.py	80.09% <50.00%> (-0.31%)	⬇️
panel/tests/pane/test_plotly.py	98.77% <0.00%> (+0.61%)	⬆️

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