chore: Change makefile python3 to python

[liuverta]

Impact and Context

I'm not sure that python3 works with Python's various virtual environment tools.

Risks and Area of Effect

Might not work if a user's environment is configured such that python is Python 2. Then again, Python 2 might not work in general more broadly. Please use Python 3.

Testing

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How to Revert

Revert this PR.