Feature: use a Python version that matches pyproject

[GuillaumeDesforges]

Under [tool.poetry.dependencies], one can set the Python version, e.g. python = "^3.8"

poetry2nix should respect that requirement.

[takeda]

Depending on implementation, this could make it hard to provide a custom python package. For example. It is right now possible to modify python package and remove non-essential builtin packages, slimming the python binary down. Or substitute with similar but not CPython (haven't tried that yet, but theoretically possible).

Perhaps this could be done when python is not specified?

[GuillaumeDesforges]

Another thing it could do is check that the requirement is met, not necessarily fix it automagically.