Replace modern pkg_resources version lookup

[Mirochill]

Summary

• use importlib.metadata.version() for package version lookup on modern
  Python installations
• preserve the existing pkg_resources lookup only as a fallback for the
  Python versions that do not provide importlib.metadata
• add coverage for the exposed package version

Rationale

Importing libcomxml currently imports pkg_resources, which emits a
deprecation warning on current Python/setuptools installations and is no
longer available in newer setuptools releases. The package still advertises
legacy Python compatibility, so this keeps its prior lookup path only where
the standard-library metadata API is unavailable.

Fixes #57

Validation

• git diff --check HEAD~1..HEAD
• static inspection of the version lookup and fallback path
• Not run locally