The packages you've install do not include the Python bindings
libxml2. You've installed the shared object library as
stated in the comments, but this is not available to Python programs
without a Python module.
python-dev libxml2 libxml2-dev libxslt-dev
sounds like you're about to build the Python module from source, so see if
the instructions you're following include other steps.
I believe this is the page for the module you need http://xmlsoft.org/python.html. See this SO answer for
instructions on how to install it on OS-X that mostly apply to Linux also