Make the elementtree XML frontend work with Python 3.3+
Make the elementtree XML frontend work with Python 3.3+

- Python 3.3 loads the C accelerated ElementTree by default. We disable
  this one temporarily, so that the pure Python version is loaded instead.
- Recent changes in ElementTree.XMLParser would break storing the XML
  source line number, so move this to TreeBuilder.

brechtm/rinohtype,brechtm/rinohtype,beni55/rinohtype,brechtm/rinohtype,beni55/rinohtype