Adjust for pint 0.18

[khaeru]

• One-line change to adjust to changes in pint.formatting.format_units() from pint 0.17 to 0.18.
• Matrix the GitHub Actions workflow to test these versions of pint, plus the latest version.

Supersedes #32 Closes #31

[codecov-commenter]

Codecov Report

Merging #35 (2224ff0) into main (7e69589) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #35   +/-   ##
=======================================
  Coverage   73.10%   73.10%           
=======================================
  Files           4        4           
  Lines         145      145           
=======================================
  Hits          106      106           
  Misses         39       39           

Impacted Files	Coverage Δ
iam_units/__init__.py	87.80% <100.00%> (ø)

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[khaeru]

@phackstock if this satisfies, let's close #32.