[deps] Add poetry support

[jjmerchante]

This PR makes the following changes:

• Change the commands from scripts to entrypoints to be in the package to support Poetry.
• Remove Python 3.6 support because the support ended on 23 Dec 2021. Now support 3.7, 3.8 and 3.9.
• Add Poetry support for installing locally and building the package.
• Fix an issue with the latest version of Git

[jjmerchante]

There is a bug in the tests, but I can't figure out why it's failing. For git 2.25 it works but for git 2.35 it fails.

It's in test_graal.py, the problem is in update() in Perceval's git backend. If I try to run the same command locally using git 2.25, it doesn't fail, but if I update the git version to 2.35, it fails.

The command is git fetch origin '+refs/heads/*:refs/heads/*' --prune

I think it must be a problem in the configuration of the tests, because executing the same command in another repository created by me works, but in the repository in the tests it fails.