Add poetry configuration

Fixes #54 (closed) Remove setup.cfg and setup.py create a __main__.py so that python -m gidocgen works Version should be changed in pyproject.toml and it is read from there in other places. Add more things .gitignore file.

For installing poetry https://python-poetry.org/docs/#installation Publishing to PyPI https://python-poetry.org/docs/cli/#publish Building Source Archives https://python-poetry.org/docs/cli/#build