BF: Use contrast in base visual, fixes #4110

[TEParsons]

based on #4109, so needs to be pulled in after

[lgtm-com]

This pull request introduces 3 alerts and fixes 1 when merging feb3e25 into 0f85cb6 - view on LGTM.com

new alerts:

• 3 for Unused import

fixed alerts:

• 1 for Unused import

[codecov]

Codecov Report

Merging #4111 (f3b5b6e) into release (0f85cb6) will increase coverage by 0.09%.
The diff coverage is 78.57%.

@@             Coverage Diff             @@
##           release    #4111      +/-   ##
===========================================
+ Coverage    35.50%   35.60%   +0.09%     
===========================================
  Files          288      288              
  Lines        56824    56751      -73     
  Branches      9698     9670      -28     
===========================================
+ Hits         20177    20205      +28     
+ Misses       34671    34575      -96     
+ Partials      1976     1971       -5     

Impacted Files	Coverage Δ
psychopy/visual/ratingscale.py	83.25% <66.66%> (+0.02%)	⬆️
psychopy/visual/helpers.py	69.67% <76.47%> (+23.56%)	⬆️
psychopy/visual/basevisual.py	67.60% <91.66%> (+2.24%)	⬆️
psychopy/colors.py	58.42% <100.00%> (+2.69%)	⬆️
psychopy/visual/shape.py	74.08% <0.00%> (+0.36%)	⬆️

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[lgtm-com]

This pull request introduces 3 alerts and fixes 1 when merging f3b5b6e into 0f85cb6 - view on LGTM.com

new alerts:

• 3 for Unused import

fixed alerts:

• 1 for Unused import