My original Jupyter install (Fedora fc27 here) was made simply by
dnf install python2-devel python2-jupyter-core python2-qtconsole python3-jupyter-core python3-qtconsole pandoc
pip install jupyter
I had a small problem: just the python2 kernel worked, not the python3 - and sometimes I needed to work in python3. So following some random link I made the error to upgrade by pip my jupyter-client, and all hell broke loose…
Now I uninstalled/reinstalled by dnf/pip a thousand times, and my problems with py3 switched side with py2 
My problem: before, when I called jupyter notebook, it launched python2.
Now instead when I do it I get:
/usr/bin/python3 -s /usr/bin/jupyter-notebook
And what is strange: opening a new python2 kernel, I have no errors, Python 2 is clearly tagged on the upper right corner, yet a simple print"Hi!" returns the usual Python3 SyntaxError: Missing parentheses in call to 'print'....