test: Use Python 3 http server
test: Use Python 3 http server

All our Debian/Ubuntu images have Python 3, and will lose Python 2 at
some point.

cockpit-project/cockpit,mvollmer/cockpit,cockpituous/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,garrett/cockpit,cockpituous/cockpit,martinpitt/cockpit,martinpitt/cockpit,deryni/cockpit,deryni/cockpit,moraleslazaro/cockpit,garrett/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,garrett/cockpit,garrett/cockpit,deryni/cockpit,deryni/cockpit,cockpit-project/cockpit,deryni/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,garrett/cockpit,deryni/cockpit,cockpituous/cockpit,deryni/cockpit,moraleslazaro/cockpit