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.

This patch does the minimum necessary to drop that support, subsequent
patches remove Python 3.6 specific workarounds separately.

Signed-off-by: John Snow <jsnow@redhat.com>