Trying to install lxml - "you have held broken packages"

Hi.
I would like to install lxml python library on my Nas (RedyNas 104) :

http://lxml.de/installation.html wrote:
To install the required development packages of these dependencies on Linux systems, use your distribution specific installation tool, e.g. apt-get on Debian/Ubuntu: sudo apt-get install libxml2-dev libxslt-dev python-dev

but when I try to install it, here is what I have :

~$ sudo apt-get install libxml2-dev libxslt-dev python-dev
Reading package lists... Done
Building dependency tree
Reading state information... Done
Note, selecting 'libxslt1-dev' instead of 'libxslt-dev'
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libxml2-dev : Depends: libxml2 (= 2.8.0+dfsg1-7+nmu2) but 2.8.0+dfsg1-7+wheezy2 is to be installed
 python-dev : Depends: python2.7-dev (>= 2.7.3-1~) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

I searched on the web to fix this problem but I didn't find anything to fix it.
Is anybody know how I can fix this problem ?

Thanks a lot.