Add tests of different pint versions

[znichollscr]

In #40 (comment) this idea was raised. It turns out to be non-trivial because Pint doesn't support Python 3.7 for newer versions. Will require adding a couple of lines to our CI CD workflow.

pint-version: [0.18, 0.19, 0.20, 0.21]

pip install 'pint==${{ matrix.pint-version }}'

[lewisjared]

I think this is unblocked now since we support Python>=3.9