Ensure lxml is installed for the tests

The lxml package is a recommends of python-pskc and is needed for
reading data from stdnum if - is supplied as file name.
6 jobs for debian/master in 7 minutes and 40 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #300073
build

00:03:12

 
  Test
passed #300077
autopkgtest

00:03:22

passed #300076
lintian

00:01:35

passed #300078
piuparts

00:04:26

passed #300075
test-build-all

00:03:18

passed #300074
test-build-any

00:02:09