Fix a type issue for windows

[kratman]

Description

This was an issue caught here: https://github.com/pybamm-team/pybammsolvers

In the standalone repo, wheels are built as part of the tests and were failing on windows. This fixed the issue.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.41%. Comparing base (444ecc1) to head (6f1acfd).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4486   +/-   ##
========================================
  Coverage    99.41%   99.41%           
========================================
  Files          293      293           
  Lines        22554    22554           
========================================
  Hits         22423    22423           
  Misses         131      131           

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