fix: make Makefile use $(PYTHON) instead of hardcoded python for python3-only systems

[corazonthedev]

Summary

Makefile targets hardcode python on PATH, but many modern environments ship
only python3. On those machines no documented commands — make test,
make ci, make demo — run at all. This issue makes the interpreter
configurable (PYTHON ?= python3) so the documented workflow works out of the
box.

Changes

File	Change
Makefile	Added PYTHON ?= python3, replaced all 67 python with $(PYTHON)
CONTRIBUTING.md	Added note documenting the PYTHON= override

Verification

• grep -cP '^\\s+python\\b' Makefile → 0 (zero raw python on command lines)
• grep -cP '^\\s+\$\(PYTHON\)' Makefile → 67 (all replaced)
• CI unaffected (uses setup-python which provides python)
• Only 2 files changed — no unrelated modifications

Closes

Closes #712

[dgenio]

Reviewed — APPROVE. Clean, complete fix for #712. Verified via make -n: the default resolves to python3, make … PYTHON=python3.11 overrides correctly, and the override propagates into the example→architectures sub-make. CI is unaffected (setup-python provides both python and python3).

Confirmed the replacement is complete — zero bare python invocations remain in the full file (checked against the head, not just the diff), and all 67 are now $(PYTHON).

Two non-blocking notes:

• CONTRIBUTING.md's pip install -e ".[dev]" bootstrap line still assumes pip on PATH (#712 mentioned the pip assumption). It's outside the Makefile-interpreter scope and usually satisfied by an active venv — flagging only for completeness.
• ruff/mypy/mkdocs are left as bare console scripts rather than $(PYTHON) -m …. That's the correct choice (not all of them support python -m); no change needed.

Acceptance criteria from #712 all met and behaviorally verified. Nice, well-scoped change.

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