Remove setup.py pkg_resources dependency

[Mirochill]

Summary

• replace the setup.py pkg_resources version checks with setuptools.__version__ and a small local version comparator
• remove the temporary setuptools<=81.0.0 build-system cap now that setup.py no longer needs pkg_resources

Fixes #320

Validation

• rg -n "pkg_resources|get_distribution|parse_version|setuptools>=42" setup.py pyproject.toml eessi -S (no setup.py pkg_resources usage remains; the remaining pkg_resources hit is the guarded namespace-package import in eessi/__init__.py)
• git diff --check HEAD~1..HEAD
• Not run locally