Fixing ‘src/lxml/etree_defs.h:14:31: fatal error: libxml/xmlversion.h: No such file or directory’

You can get this error if you try to install lxml library:
As simple as:

yum install libxslt-devel libxml2-devel

for Centos or

apt-get install libxml2-dev libxslt1-dev

for Ubuntu.