First of all install the darn pyserial
pip3 install pyserial
If you see this error - again- then it's 100% because is a python c*ck up as /usr/bin/python points to the python2.x not the python3 binary.
solution (tested on OpenSUSE) :
sudo rm /usr/bin/python
sudo ln -s /usr/bin/python3 /usr/bin/python
