While installing Readthedocs package inside Virtual Environement on Ubuntu 14.04 LTS , I found this error “Python headers are required to build Mercurial but weren’t found in /usr/include/python2.7/Python.h” . It seems the dependency issue with header files for python.

So what should be fix for this issue. It is very simple, we have to installed the python-dev package .

sudo apt-get update
sudo apt-get install python-dev

Now try installing the packages again, I hope it will fix your issue too.