Correctly specify python version

[micwoj92]

From: https://setuptools.pypa.io/en/latest/userguide/dependency_management.html#python-requirement

pyproject.toml requires dash, where underscore is used in case of setup.py or setup.cfg

This prevented from building using setuptools:

ValueError: invalid pyproject.toml config: `project`.
configuration error: `project` must not contain {'requires_python'} properties

Where using flit backend it would give message:

Unexpected names under [project]: requires_python