libxml2: Port gentest.py to Python-3

* but it still won't work well on hosts without libxml2, make
  sure to use pre-generated testapi.c in do_compile_ptest

* this is reproducible with SOURCE_DATE_EPOCH set to 0 which
  e.g. meta-updater still sets by default for DISTROs which
  use it :(, see https://github.com/uptane/meta-updater/pull/35



Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>