Use lxml, a fast XML parser

See the commit log for details. But basically it makes a huge difference on a raspberry-pi.

But, the question is: how to depend on lxml that are python bindings for some C library ?

Taking the approach that's being used now (including on site-packages) is not that easy, as the C library needs to be compiled for the architecture it will run. And even if we do have a C library compiled for all arch that will work, I'm not sure how to make the trick to choose the right one on each architecture.

Do you have any idea on how to handle this ?

It will be nice to have a fast XML parser