Add support for poetry as an optional, alternative packaging backend

[max-moser]

pipenv has a history of... being difficult at times.poetry is the newish kid on the block, and looks quite promising.
I've had scenarios where poetry was able to lock the dependencies within seconds or minutes, while pipenv couldn't come up with any results in an hour.
As such, it would be nice to have the option to work with poetry rather than pipenv (if you know what you're doing).

This PR adds support for poetry for most of the operations as an alternative packaging tool.
One known issue in invenio-cli is that poetry show --outdated will always exit with a status code of 0, and thus invenio-cli will always report that all packages are up to date (even though the previous output clearly suggested the opposite).
Further, the handling of pre-release package versions is different between pipenv and poetry: It seems that poetry only accepts an --allow-prereleases flag when installing additional packages, but neither on locking nor installing dependencies from the lock file (which I feel makes sense).
Another notable difference is that poetry does not, by itself, read the environment variables from a .env file. This would have to be done in code, likely via python-dotenv before executing poetry commands.

Note: By default, invenio-cli will still use pipenv.
The configuration ([cookiecutter]) has to request packaging_backend = poetry specifically to switch to poetry.

[apirogov]

Just dropping by to say yes, please!

We're using poetry for our stuff, if there would be a way to get pipenv out of the equation and stick with one build tool, it would be great!

[max-moser]

funny, i was thinking about closing this PR just a few days back 😄
but if there's still interest in this, i can have a further look

[apirogov]

We'd be quite happy to beta-test the "experimental" poetry-based instance setup once this is merged :)

At least I have some more experience dealing with the quirks of poetry, while all of us don't have experience with pipenv, adding up to the layers of abstraction Invenio introduces and we are just learning to navigate... 😅