Add url for pint in README

[blackary]

• [ ] Closes # (insert issue number)
• [ ] Executed pre-commit run --all-files with no errors
• [ ] The change is fully covered by automated unit tests
• [ ] Documented in docs/ as appropriate
• [ ] Added an entry to the CHANGES file

[blackary]

@andrewgsavage what do you think of this change? I personally came across this library and had to google for a while before I could figure out what it actually did, so it seems like it would be nice to have a link and/or description of pint on the README for those unfamiliar with it.

[andrewgsavage]

bors r+

[bors[bot]]

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