[FIX] owkaplanmeier: fix crashing legend

[JakaKokosar]

Issue

Fixes: #40
Kaplan-Meier legend is crashing due to biolab/orange3#5680

Description of changes

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

Merging #41 (0974718) into master (f25c3ab) will increase coverage by 0.32%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   45.25%   45.57%   +0.32%     
==========================================
  Files          12       12              
  Lines        1169     1176       +7     
==========================================
+ Hits          529      536       +7     
  Misses        640      640              

Impacted Files	Coverage Δ
...contrib/survival_analysis/widgets/owkaplanmeier.py	94.30% <100.00%> (+0.09%)	⬆️

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