build: bump minimum python version to 3.9

[thegamecracks]

This bumps the minimum Python version to 3.9 to prevent installing on older versions where the dict[] and list[] typehint syntax (PEP 585) is unsupported.

Even if merged and released now though, I believe pip install opengsq will automatically install older versions where the package alleges its compatibility, e.g. v0.1.0 to v3.5.0. v3.2.0 is the latest version I could get to run on Python 3.8, having briefly tested it by running py -m opengsq.protocols.minecraft.

Perhaps a hotfix could also be released that contains the incorrect version specifier and raises an ImportError if they're on Python <3.9, clearly explaining to the user that they should downgrade to opengsq 3.2.0? That should catch anyone on 3.8 or older and help guide them to the correct opengsq version, as long as they haven't pinned their opengsq version to exactly 3.3.0-3.5.0.