Python 3 compat

[ellert]

The test gives warnings with python 3.

-- Found Doxygen: /usr/bin/doxygen (found version "1.8.13") found components:  doxygen missing components:  dot
-- Checking for package 'Sphinx'
-- Found PythonInterp: /usr/bin/python3.6 (found version "3.6.7") 
  File "<string>", line 1
    import sphinx; print sphinx.__version__
                              ^
SyntaxError: invalid syntax
-- Found Sphinx: /usr/bin/sphinx-build  

With the PR:

-- Checking for package 'Sphinx'
-- Found PythonInterp: /usr/bin/python3.6 (found version "3.6.7")
-- Found Sphinx: /usr/bin/sphinx-build

[gbitzes]

Thanks!