Change parser to lxml and use mkaurball

Hi Fabien,
I faced some issues with aury today:
- first a parsing issue:
```
xml.etree.ElementTree.ParseError: not well-formed (invalid token)
```
I didn't found a solution to this with ElementTree, so I gave a try with lxml which is working fine. It adds a dependency (actually two, I forgot to add `python-cssselect` to the PKGBUILD), but it is easier to use and more robust.

- then burp refused to push my package with a message about `.AURINFO`. 
https://wiki.archlinux.org/index.php/AURINFO
So I have added `pkgbuild-introspection` to use `mkaurball` instead of `makepkg --source`.
I can split this commit to another PR if you don't want the lxml dependency ;-).