use poetry instead of requirements.txt

[zhukovgreen]

Poetry (https://github.com/python-poetry/poetry) is python modern dependency management tool, which is compliant with PEP 517 (pyproject.toml).

Advantages of using poetry for our project:

1. Dependencies resolver for multiple python environments as a separate set of dependencies versions (no need to find the max common between py2 and py3 versions)
2. Dependencies are locked with the special lock file which ensures 100% matching python environments installed in different systems and in different time
3. pyproject.toml support allows us to have all configurations just in one file (pytest, isort, black, ex setup.py) (with one exception - flake8 still not supports pyproject)

Some examples of workflows with the poetry:

$ poetry install    # create virtual env and install project dependencies inside
$ poetry update   # updates all dependencies inside all environment to the most recent available versions respecting the deps constraints
$ poetry env use 2.7 && poetry run pytest # run pytest in py2 environment
$ poetry env use 3.8 && poetry run pytest # run pytest in py3 environment

Example of the poetry setup inside pyproject.toml:

[tool.poetry]
name = "leapp-repository"
version = "0.0.1"
description = ""
authors = ["ZhukovGreen <[email protected]>"]

[tool.poetry.dependencies]
python = "~2.7 || ^3.6"
#leapp = {git = "https://github.com/oamg/leapp.git"}
leapp = {path = "../leapp", develop = true}
zipp = {version = "^1.2", python = "~2.7"}
ipaddress = {version = "^1.0.23", python = "~2.7"}
distro = "^1.5.0"

[tool.poetry.dev-dependencies]
coverage = {extras = ["toml"], version = "^5.0.3"}
pathlib2 = {version = "^2.3", python = "~2.7"}
pytest = [
    {version = "^4.6.11", python = "~2.7"},
    {version = "^6.0.0", python = "^3.6"}

]
pytest-mock = [
    {version = "^2.0.0", python = "~2.7"},
    {version = "^3.1.1", python = "^3.6"}
]
pytest-cov = "^2.8.1"
pyudev = "^0.22.0"
ipdb = "^0.13.2"
mock = [
    {version = "^3.0.5", python = "~2.7"},
    {version = "^4.0", python = "^3.6"}
]
pylint = [
    {version = "^1.9", python = "~2.7"},
    {version = "^2.4.4", python = "^3.6"}
]
flake8 = "^3.7.9"
pytest-xdist = "^1.33.0"

[build-system]
requires = ["poetry>=1.0.0"]
build-backend = "poetry.masonry.api"

Under the scope:

• add poetry setup to the pyproject.toml and remove requirements.txt
• update Makefile to use poetry
• update ci to use poetry

resolves #504