fix mpu.colorbar for matplolib v3.4 and newer

[mathause]

• Closes fix compatibility of mpu.colorbar with mpl >= 3.4 #6

The main difference is the usage of set_box_aspect instead of set_aspect. And also the correct computation of the position (this was simplified before and relied on mpl bbox_inches="tight" to cut them off).

[codecov-commenter]

Codecov Report

Base: 65.50% // Head: 65.74% // Increases project coverage by +0.24% 🎉

Coverage data is based on head (3b82d1b) compared to base (b9048b8).
Patch coverage: 93.10% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   65.50%   65.74%   +0.24%     
==========================================
  Files           4        4              
  Lines         316      327      +11     
==========================================
+ Hits          207      215       +8     
- Misses        109      112       +3     

Flag	Coverage Δ
min-all-deps	?
unittests	65.74% <93.10%> (+0.24%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mplotutils/colorbar_utils.py	81.25% <93.10%> (+2.04%)	⬆️
mplotutils/__init__.py	61.53% <0.00%> (-23.08%)	⬇️

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