ci: drop python 3.7 and 3.9
ci: drop python 3.7 and 3.9

Testing with older (3.6) and newer (3.10) should be enough
and covers the versions shipped by Ubuntu supported releases.

Signed-off-by: Simon Deziel <eca3f1a14b730e70ce4898287e4fc2f9966fa3a6@canonical.com>

lxc/pylxd,lxc/pylxd