Pin poetry version in pyproject.toml

[adubovik]

Consider defining poetry version in pyproject.toml as a single source of truth:

[build-system]
requires = ["poetry-core==1.6.1"]
build-backend = "poetry.core.masonry.api"

[tool.poetry]
requires-poetry = "==1.6.1"

Modify Makefile and README is needed accordingly.

Make sure that this approach doesn't force the user to install any particular version of poetry globally.