core_text: drop dependency on PyXML

use sax.xml API which gives us the first parser available, instead of
using a specific parser that happens to come from pyxml
Status Job ID Name Coverage
  External
running ERP5.CodingStyleTest-Master

19680:00:47

running ERP5.UnitTest-Master

19680:00:42

running ERP5.UnitTest-Master.Medusa

19680:00:44

passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:09

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:27