Fix deprecation warnings raised by tests

[niksirbi]

The tests were catching some DeprecationWarniings about some specific syntax elements that will not be supported in future versions of numpy and matplotlib.

In the process of fixing the matplotlib warning, I also discovered a missing colorspace conversion from RGB to BGR (needed for cv2 to properly display colors in the render plots). I've fixed that, but it may no longer be relevant if we decide to drop cv2, as suggested in #117.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.

Project coverage is 55.26%. Comparing base (ddcd22a) to head (a69e156).

Files with missing lines	Patch %	Lines
neuralplayground/arenas/discritized_objects.py	88.88%	1 Missing ⚠️
neuralplayground/arenas/simple2d.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
+ Coverage   55.18%   55.26%   +0.07%     
==========================================
  Files          40       40              
  Lines        3441     3447       +6     
==========================================
+ Hits         1899     1905       +6     
  Misses       1542     1542              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.