hotfix box plotting to support latest pandas set_categories

[gidden]

With latest pandas, plot.box() now borks if you use the by kwarg (see here)

Simple fix in this PR

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.0%. Comparing base (ddbb88e) to head (af2022b).
Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
pyam/plotting.py	50.0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main    #891    +/-   ##
======================================
  Coverage   95.0%   95.0%            
======================================
  Files         64      64            
  Lines       6134    6242   +108     
======================================
+ Hits        5828    5936   +108     
  Misses       306     306            

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