fix: make Makefile Python path portable

[jfilipcz]

Description

Make the Makefile Python executable portable by replacing the hardcoded /opt/homebrew/bin/python3.13 path with an overridable
PYTHON ?= python3 default. This lets contributors use the active python3 while still allowing explicit overrides such as make PYTHON=/path/to/python test.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test improvement

Testing

• Tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested the changes manually

Tested with:

• make -n test
• make PYTHON=/tmp/himl-analysis-venv/bin/python test

Result: 197 passed

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

Additional Notes

This change keeps the existing Makefile workflow intact while removing a workstation-specific Python path that fails on other
environments.