Updates for recent Pillow changes

[saimn]

Fix #474

[codecov]

Codecov Report

Merging #479 (ce40d84) into main (35d42e9) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #479      +/-   ##
==========================================
- Coverage   88.13%   88.02%   -0.12%     
==========================================
  Files          24       24              
  Lines        2057     2062       +5     
==========================================
+ Hits         1813     1815       +2     
- Misses        244      247       +3     

Impacted Files	Coverage Δ
sigal/gallery.py	91.24% <100.00%> (+0.01%)	⬆️
sigal/image.py	91.73% <100.00%> (-0.74%)	⬇️
sigal/plugins/watermark.py	86.04% <0.00%> (-2.33%)	⬇️

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