chore: Relax python version req. to include 3.9

[ColbyDeLisle]

Title, basically. The current set of dependencies allow for relaxing the Python dependency in the pyproject.toml down to >=3.9. I was hoping to go all the way down to >=3.7, since that is what pyzx requires, but this was not possible without downgrading other dependencies.

[ABorgna]

Thanks! We may need to add the python versions to the package classifiers before publishing to pypi, but for now this should be enough.