Fix DeprecationWarning

[akiomik]

Fix DeprecationWarning of #16

[codecov]

Codecov Report

Merging #18 (afbe20c) into master (043492c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #18   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           62        62           
  Lines          778       778           
=========================================
  Hits           778       778           

Impacted Files	Coverage Δ
pilgram/css/blending/soft_light.py	100.00% <100.00%> (ø)

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