Avoid pkg_resources for package version

[Mirochill]

Summary

• replace pkg_resources.require() with importlib.metadata.version() for unitpy.__version__
• keep the package version single-sourced from installed distribution metadata without requiring pkg_resources at runtime

Fixes #16

Validation

• Not run locally
• git diff --check HEAD~1..HEAD
• rg 'pkg_resources' (no matches in project files)