Remove Python 2 compat: six, cached-property, __future__ imports

[iskandr]

Summary

• Remove six dependency — replace with Python 3 builtins (str, int, bytes, range, zip, metaclass syntax)
• Remove cached-property — use functools.cached_property (stdlib since 3.8)
• Remove from __future__ import statements from all files
• Add python_requires='>=3.9'
• Bump version to 1.4.0

Test plan

• CI passes on Python 3.9, 3.10, 3.11