error when installing LXML for Python on mac with OSX 10.10 Yosemite

Question

I can't manage to install LXML on a mac with OSX 10.10.3 Yosemite, in Python 2.7.9.

I run the magic command:

STATIC_DEPS=true sudo pip install lxml

suggested here and here

but when I import lxml.html I get this error:

Traceback (most recent call last):
  File "<pyshell#22>", line 1, in <module>
    import lxml.html
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/lxml/html/__init__.py", line 42, in <module>
    from lxml import etree
ImportError: dlopen(/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/lxml/etree.so, 2): Symbol not found: _exsltDateXpathCtxtRegister
  Referenced from: /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/lxml/etree.so
  Expected in: flat namespace
 in /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/lxml/etree.so

similar problems also reported in other questions, this and this

I had no luck. I tried to reinstall everything but it always tell me

Requirement already satisfied (use --upgrade to upgrade): lxml in /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages
Cleaning up...

I have no idea how to fix this, can anyone help me?