Update for Pint 0.20 API breaks

[mattwthompson]

Description

This PR attempts to update QCElemental to work with the newest version of Pint. Version 0.20.0 moved around a ton of stuff internally, although the only breakage here was the disappearance of an easily accessible Quantity class. Somewhat confusingly type(Quantity) returns something that does not obviously exist, but importing directly from the top-level namespace does work.

>>> import pint
>>> ureg = pint.UnitRegistry()
>>> type(ureg.Quantity("1 angstrom"))
<class 'pint.util.Quantity'>
>>> from pint.util import Quantity
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: cannot import name 'Quantity' from 'pint.util' (/Users/mattthompson/mambaforge/envs/test/lib/python3.11/site-packages/pint/util.py)
>>> from pint import Quantity
>>> isinstance(ureg.Quantity("1 angstrom"), Quantity)
True

Prior to seeing a similar attempt from @loriab, I had this to say, which is not quite correct:

I believe this comes from the code around here - in any case one cannot simply import any sort of Quantity as it is no longer a standalone class but something that dangles off of the registry. Fortunately the change here appears to be simple - the internals were not reached into too deeply and the change to one bit of input sanitization didn't need information it didn't already have in scope.~~

Changelog description

Update for compatibility with Pint 0.20.

Status

• Code base linted - I'd be happy to move this to use https://pre-commit.ci/ so users never need to lint themselves
• Ready to go

[codecov]

Codecov Report

Merging #296 (e323061) into master (2956421) will decrease coverage by 0.17%.
The diff coverage is 71.42%.

Additional details and impacted files