Use poetry for packaging and dependency management

Use poetry as a packaging and dependency management tool. It replaces requirements*.{in,txt} files with pyproject.toml (the new standard file for python project configuration) and poetry.lock (lock file, specific to poetry.)

Poetry also automatically creates per-project virtual environements, so it can replace tox as well.

This config defines a python package that can be published to pypi, named sigsum-witness, so that we can eventually offer pip install sigsum-witness as an installation method.

To be able to installs a sigsum-witness script with poetry, sigsum-witness.py was renamed sigsum_witness.py.