Adapt to matplotlib 3.5

[dweindl]

Current code fails due to changes in matplotlib. This fixes it.

[codecov-commenter]

Codecov Report

Merging #86 (e0a09f8) into develop (9047f2b) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff            @@
##           develop      #86   +/-   ##
========================================
  Coverage    79.59%   79.59%           
========================================
  Files           26       26           
  Lines         2940     2940           
  Branches       695      695           
========================================
  Hits          2340     2340           
  Misses         423      423           
  Partials       177      177           

Impacted Files	Coverage Δ
petab/visualize/plotter.py	78.03% <50.00%> (ø)
petab/visualize/plotting_config.py	68.04% <50.00%> (ø)

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