Support Pint 0.20 and Pint 0.21

[quentinmit]

Pint 0.20 removes the (unsupported) pint.unit and pint.converters packages. Pint 0.21 adds a native percent unit.

This PR checks if the percent unit exists, and defines it in a Pint 0.19+0.20-compatible way if it does not.

BTW, in general you shouldn't use == version dependencies in a library like this, because it can conflict with downstream users of the library. Only end-user apps should have exact version dependencies.

[natekspencer]

This PR checks if the percent unit exists, and defines it in a Pint 0.19+0.20-compatible way if it does not.

Awesome, thanks!

BTW, in general you shouldn't use == version dependencies in a library like this, because it can conflict with downstream users of the library. Only end-user apps should have exact version dependencies.

Where are you seeing a == version constraint?

[quentinmit]

BTW, in general you shouldn't use == version dependencies in a library like this, because it can conflict with downstream users of the library. Only end-user apps should have exact version dependencies.

Where are you seeing a == version constraint?

Sorry, I should have said "^ version dependencies with a minor version" instead of just ==. Basically, the previous Pint = "^0.19" meant that it couldn't be used with any other minor version of Pint. In this case 0.20 wasn't compatible without this patch, but in general that won't be the case and the ^ dependency will be the only thing stopping someone from combining this library with other code that wants a newer version of Pint/another library.

[natekspencer]

Ahh thanks for the clarification, and yeah that makes sense!