use ElementTree

cElementTree
--------------------
The advantages are obvious. (massive speed improvements and smaller memory footprint)
cElementTree is included in the most linux distribution and the python 4 windows distribution have it too.

ElementTree
------------------
shipped with python standard library since version 2.5.

SimpleXMLWriter
-------------------------
Is not really needed. If some changes are made​​, this dependence can be removed.

Why remove lib/elementtree from B3?
- Python provides everything needed
- The code's ancient
- The maintenance will be obsolete