Fix setup-python in GitHub Actions

[eytanadler]

Purpose

In #56, I believe setup-python@v2 was accidentally changed to checkout@v4 rather than setup-python@v4. This PR fixes that.

Expected time until merged

Soon, since this affects the builds of all repos that use GitHub Actions (the flake8 action is probably the most pressing).

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

Checklist

• I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation