Fix pint regression

[znichollscr]

Pull request

Please confirm that this pull request has done the following:

• Tests added
• Description in CHANGELOG.rst added

Adding to CHANGELOG.rst

Please add a single line in the changelog notes similar to one of the following:

- (`#XX <https://github.com/openscm/openscm-units/pull/XX>`_) Added feature which does something
- (`#XX <https://github.com/openscm/openscm-units/pull/XX>`_) Fixed bug identified in (`#YY <https://github.com/openscm/openscm-units/issues/YY>`_)

[znichollscr]

Urgh @mikapfl and @lewisjared this is a super annoying regression in Pint. Despite this quite sensible comment (hgrecco/pint#1277 (comment)), ppm was added to Pint anyway (hgrecco/pint#1277). This broke things.

The fix here works, but it means you have to rebuild the cache which just feels painful. I can't see how else to get around it though given this hgrecco/pint#740

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f3c7ab1) 100.00% compared to head (cee2c7f) 100.00%.

❗ Current head cee2c7f differs from pull request most recent head 90f835b. Consider uploading reports for the commit 90f835b to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #40   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          123       124    +1     
  Branches        18        18           
=========================================
+ Hits           123       124    +1     

Impacted Files	Coverage Δ
src/openscm_units/_unit_registry.py	100.00% <100.00%> (ø)

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[znicholls]

Merging, we can deal with pint hell as part of #42