setup assumes that sphinx-build is present in /usr/bin which is not always the case

In build-with_docs method in setup.py it is assumed that sphinx-build will always be present in /usr/bin i.e. $prefix is always /usr/

In case sphinx is installed locally it will go in /usr/local unless --prefix=/usr/ is explicitly passed when building sphinx.
In other situation sphinx can also be present in /opt

It is better to scan where sphinx is present by getting the PATH environment varible, spitting it by ; and then appending sphinx-build and checking if it exists. Do note that /usr/local/bin has higher priority than /usr/bin/