:heavy_plus_sign: Use Poetry for package management (#144)

* use poetry insted of Pipfile

* fix python black version

* set prepare.sh as executable

* revert postgres 11

* use multi-build stage in docker

* fix poetry path

* :fire: Remove uneeded changes

* :wrench: Move and update Poetry file

* :see_no_evil: Update gitignore

* :whale: Update Dockerfiles to use Poetry

* :whale: Update Dockerfiles with Poetry

* :wrench: Add SERVER_NAME required by Celery worker

* :whale: Update Poetry install to avoid env conflicts

* :heavy_plus_sign: Add Pytest to Poetry dependencies

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>