Add `<PACKAGE>.__version__` functionality

[lawrenceabird]

Add functionality to retrieve the "version" for the installed package.

Add this to the PACKAGE-level __init__.py

import importlib.metadata

try:
    __version__ = importlib.metadata.version("PACKAGE")
except importlib.metadata.PackageNotFoundError:
    __version__ = "0.0.0"

This pulls the version number from the pyproject.toml, so it's consistent with that used for a given release. If it's read from source (e.g. in development state), it falls back to "0.0.0"

[lawrenceabird]

This was addressed in #30