Update to new openscm-units API

[znicholls]

Pull request

Please confirm that this pull request has done the following:

• Tests added (N/A)
• Documentation added (where applicable) (N/A)
• Example added (either to an existing notebook or as a new notebook, where applicable) (N/A)
• 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/GranthamImperial/silicone/pull/XX>`_) Added feature which does something
- (`#XX <https://github.com/GranthamImperial/silicone/pull/XX>`_) Fixed bug identified in (`#YY <https://github.com/GranthamImperial/silicone/issues/YY>`_)

[znicholls]

Solves our failing pypi install (https://github.com/GranthamImperial/silicone/actions/runs/439203228) once we do a new release