Upgrade supported Python versions

[ConorMacBride]

• Drop support for py37 and add support for py310 and py311
• Use Python Limited API so older wheels hopefully automatically work with newer versions of Python
• Upgrade Matplotlib's get colormap API to fix a warning
• Upgrade the Matplotlib version used for the figure tests

[codecov]

Codecov Report

Merging #49 (ef17df3) into main (2443a06) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
+ Coverage   95.06%   95.07%   +0.01%     
==========================================
  Files          18       18              
  Lines        1378     1382       +4     
==========================================
+ Hits         1310     1314       +4     
  Misses         68       68              

Impacted Files	Coverage Δ
src/mcalf/utils/plot.py	100.00% <100.00%> (ø)
src/mcalf/visualisation/velocity.py	100.00% <100.00%> (ø)

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