Make Makefile Python targets use python3

[CooperSheroy]

Summary

• add a configurable PYTHON variable that defaults to python3
• run pip through $(PYTHON) -m pip for interpreter consistency
• fix the lint target's composition validator path

Validation

• make lint
• git diff --check

[rajpratham1]

This is a solid cross-platform improvement. Replacing hardcoded python commands with $(PYTHON) makes the Makefile more portable and respects the user's configured Python interpreter. Using python3 -m pip is also a recommended practice since it ensures packages are installed into the same interpreter that's executing the project. The small lint path correction (tools/composition_validator.py → tools/analysis/composition_validator.py) also appears consistent with the current project structure.

[calesthio]

Approved.. Solid PR