I am trying to do source installation of Odoo 9 on Ubuntu 14.04

Following the documentation of Odoo (https://www.odoo.com/documentation/9.0/setup/install.html#setup-install-source)

In the step :

$ pip install -r '/home/invendoi/odoo/doc/requirements.txt'

I got the following error

invendoi@invendoi:~$ pip install -r '/home/invendoi/odoo/doc/requirements.txt'

Downloading/unpacking sphinx==1.2.3 (from -r /home/invendoi/odoo/doc/requirements.txt (line 1))

Downloading Sphinx-1.2.3-py2-none-any.whl (1.9MB): 1.9MB downloaded

Downloading/unpacking mercurial==3.2.2 (from -r /home/invendoi/odoo/doc/requirements.txt (line 2))

Downloading mercurial-3.2.2.tar.gz (4.1MB): 4.1MB downloaded

Running setup.py (path:/tmp/pip_build_invendoi/mercurial/setup.py) egg_info for package mercurial

Python headers are required to build Mercurial but weren't found in /usr/include/python2.7/Python.h

Complete output from command python setup.py egg_info:

running egg_info

creating pip-egg-info/mercurial.egg-info

writing pip-egg-info/mercurial.egg-info/PKG-INFO

writing top-level names to pip-egg-info/mercurial.egg-info/top_level.txt

writing dependency_links to pip-egg-info/mercurial.egg-info/dependency_links.txt

writing manifest file 'pip-egg-info/mercurial.egg-info/SOURCES.txt'

warning: manifest_maker: standard file '-c' not found

Python headers are required to build Mercurial but weren't found in /usr/include/python2.7/Python.h

----------------------------------------

Cleaning up...

Command python setup.py egg_info failed with error code 1 in /tmp/pip_build_invendoi/mercurial

Storing debug log for failure in /home/invendoi/.pip/pip.log

invendoi@invendoi:~$

What is the reason behind that ?