Update test PyPI

[znicholls]

Description

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2412e66) to head (424fccf).
⚠️ Report is 158 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #15   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           18        18           
=========================================
  Hits            18        18           

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[znicholls]

An example of a test run: https://github.com/openscm/example-fgen-basic/actions/runs/17070469205

As expected, installation from wheel fails (because there are no wheels, x-ref #10) but installing from source works, which is good