Python 3.8 is EOL and no longer receives security upgrades.

The changes here include:

1. Updating documentation
2. Updating dependency specifications
3. Improving the search for LLVM (toe stubbed during development)
4. Removing 3.8-specific special cases from GHA

This also enforces a dependency management scheme where the pyproject.toml file encodes known constraints, which are satisfied by the locked versions in requirements.txt.