Convert project to a Poetry project

Convert project to a Poetry project

Poetry <https://python-poetry.org/> handles the build creation and
`setup.py` and `PKG-INFO` generation.

Poetry also handles dependencies and can create a corresponding virtual
environment having them installed.

Poetry can also publish.

`poetry.lock` was generated by `poetry install` and ensures this
specific version of barectf is tested with those specific versions of
the dependencies.

Setuptools is a run-time dependency for its `pkg_resources` package. I
use `*` for its version constraint because somehow Poetry needs
Setuptools itself so it skips this dependency (see
<https://github.com/python-poetry/poetry/issues/1584>). The `setuptools`
dependency still ends up in `setup.py`.

`tox.ini` is calibrated so that tox doesn't manage any dependency;
everything is done through Poetry (except `flake8` which doesn't need
barectf to be installed).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>