Enhancement: stop using pkg_resources

[GideonBear]

From the setuptools documentation:

Use of pkg_resources is deprecated in favor of importlib.resources, importlib.metadata and their backports (importlib_resources, importlib_metadata). Users should refrain from new usage of pkg_resources and should work to port to importlib-based solutions.

python -Wall -c 'import docxcompose.properties' outputs:

.../venv/lib/python3.9/site-packages/pkg_resources/__init__.py:121: DeprecationWarning: pkg_resources is deprecated as an API
  warnings.warn("pkg_resources is deprecated as an API", DeprecationWarning)