Replace pkg_resources version lookup

[Mirochill]

Summary

• Replace the pkg_resources.get_distribution(pew) lookup used by pew version with importlib.metadata.version.
• Add the importlib-metadata backport for Python <3.8 to preserve the existing Python 3.6/3.7 support targets.
• Add unit coverage for version output and the missing package metadata fallback.

Fixes #242

Validation

• git diff --check HEAD~1..HEAD
• rg -n pkg_resources|DistributionNotFound|resource_filename|load_entry_point pew tests setup.py requirements.txt (no matches)
• Not run locally: test suite