Drop Python3.6 support
Python 3.6 was EOL as of 2021-12-23; this project carried support for an additional period of time for the sake of QEMU's support of RHEL 8 and OpenSUSE Leap, but can no longer be supported in addition to 3.12 and modern tool ecosystems on e.g. Fedora 39's downstream.
Specifically, commit b5aa6830 mentions that for Python 3.6's sake we needed to pin against a lower version of setuptools_scm. Fedora 39 no longer offers a version of setuptools_scm low enough, so we must drop Python 3.6 support and lift the requirement.