fix: declare the supported Python runtime

[ryanduguid]

Summary

• raise the declared Python floor from 3.5 to 3.8, matching the existing setuptools>=75.1.0 runtime constraint
• advertise and configure tox for Python 3.8 through 3.13; companion CI PR ci: test the Python event source #241 executes the exact floor and ceiling
• stop producing a deprecated Python 2/3 universal wheel for this Python 3-only package

Validation

• parsed all 609 package and test modules with Python 3.8 grammar
• companion CI PR ci: test the Python event source #241 adds hosted execution on Python 3.8, 3.12 and 3.13 and builds the package on 3.13; that exact-head run is required evidence before treating 3.13 as supported
• built both sdist and wheel with python -m build
• verified wheel metadata contains Requires-Python: >=3.8
• verified the wheel tag is py3-none-any
• git diff --check

PyPI metadata for setuptools 75.1.0 declares Python 3.8 or newer, so the former >=3.5 package metadata could promise an installation that its own dependency list rejects. This metadata PR remains draft pending the companion hosted runtime matrix.