Python 3.13 support in validator.py

[pdecks]

Fixes #1173

Why This Broke in Python 3.13
Python ≤ 3.12: When exec(func) was called with only globals, the dynamically generated generate_validate_all function was accessible in the local scope, and the fallback locals()['generate_validate_all'] worked.

Python 3.13: The scoping behavior changed. When exec() is called with only globals, the generated function is no longer accessible via locals(), causing the KeyError: 'generate_validate_all' encountered in #1173.

[CLAassistant]

All committers have signed the CLA.

[pdecks]

PTAL @rodaine 😎

[rodaine]

Thanks, @pdecks!