travis: Test on Python 3.6–3.9
travis: Test on Python 3.6–3.9

Drop support for Python < 3.6, as it’s no longer supported upstream.

Signed-off-by: Philip Withnall <12d956b70be7f0d60e54e48e97a01dac38dce989@endlessos.org>

pwithnall/dbus-deviation,pwithnall/dbus-deviation,pwithnall/dbus-deviation