src/poky: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 uptane/meta-updater#35

(From OE-Core rev: 178cea1593dc6e9a7eb74842615356d90d79f78f)

Signed-off-by: Steve Sakoman <[email protected]>
Signed-off-by: Martin Jansa <[email protected]>
Signed-off-by: Richard Purdie <[email protected]>