Fix setup.py to not use pip internal APIs

[JustAnalyze]

Problem

The current setup.py imports from pip.req and pip._internal.req, which are internal pip APIs that should not be used. This causes ModuleNotFoundError: No module named 'pip' when building in PEP 517 isolated environments (used by modern build tools like uv, pip>=20, build, etc.).

Solution

• Read requirements.txt directly using standard Python file I/O
• Switch from distutils to setuptools (distutils is deprecated in Python 3.10+)
• Remove all pip internal API usage
• Add python_requires field for better package metadata

Testing

Tested with:

• ✅ pip install .
• ✅ uv pip install .
• ✅ python -m build

All installation methods work correctly after this fix.

Related Issues

This fixes issues when using modern Python packaging tools that use PEP 517 isolated builds.