fix PIL deprecation warning

[lanpa]

https://pillow.readthedocs.io/en/stable/releasenotes/9.1.0.html

found these discussions:
python-pillow/Pillow#6684
python-pillow/Pillow#6537
python-pillow/Pillow#5954

[codecov-commenter]

Codecov Report

Base: 78.96% // Head: 79.00% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (3f21f7b) compared to base (52f8bd2).
Patch coverage: 80.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #677      +/-   ##
==========================================
+ Coverage   78.96%   79.00%   +0.03%     
==========================================
  Files          39       39              
  Lines        2677     2681       +4     
==========================================
+ Hits         2114     2118       +4     
  Misses        563      563              

Impacted Files	Coverage Δ
tensorboardX/summary.py	86.71% <80.00%> (-0.16%)	⬇️
tensorboardX/x2num.py	63.26% <0.00%> (+2.04%)	⬆️

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