Installation: Install sphinx with pip for python3(pip3 like that).
pip3 install -U sphinx
Building: Makefile
(linux/Mac) changes.
SPHINXBUILD = python -msphinx
In above line in Makefile change python to python3(or python3.x) like
SPHINXBUILD = python3 -msphinx
if default python is pointing to 2.x version
python.