Fix Python 3.12 compatibility by upgrading setuptools requirement

[nicklecoder]

Summary

Fixes #5

This PR resolves the AttributeError: module 'pkgutil' has no attribute 'ImpImporter' error that occurs when installing faststylometry on Python 3.12+.

Root Cause

Python 3.12 removed the deprecated pkgutil.ImpImporter class. The previous setuptools requirement (>=46.4.0) allowed versions that still reference this removed class during the build process, causing installation failures.

Changes

• Updated setuptools requirement from >=46.4.0 to >=66.1.0 in pyproject.toml

Why 66.1.0?

This is the earliest setuptools version that fixes the pkgutil.ImpImporter issue (setuptools#3685). Using the minimum compatible version provides better backwards compatibility.

Impact

This eliminates the need for the numpy downgrade workaround currently documented in the README (lines 44-64).

Testing

• ✅ Successfully tested installation on Python 3.12.3 with setuptools 66.1.0
• ✅ Package builds without errors
• ✅ Package imports correctly
• ✅ Basic functionality verified
• ✅ All dependencies install without errors

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