Since I found no answer for my question, neither in older posts nor in other forums, I want to ask the stackoverflow community for advice.
I am using a raspberry pi 3B
Today I found the solution. The steps below worked for me, without any error. The whole process took almost two hours.
sudo apt-get update
sudo apt-get install qt5-default
sudo apt-get install sip-dev
cd /usr/src
sudo wget https://www.riverbankcomputing.com/static/Downloads/sip/sip-4.19.14.tar.gz
sudo tar xzf sip-4.19.14.tar.gz
cd sip-4.19.14
sudo python3.6 configure.py --sip-module PyQt5.sip
sudo make
sudo make install
cd /usr/src
sudo wget https://www.riverbankcomputing.com/static/Downloads/PyQt5/PyQt5_gpl-5.12.tar.gz
sudo tar xzf PyQt5_gpl-5.12.tar.gz
cd PyQt5_gpl-5.12
sudo python3.6 configure.py
sudo make
sudo make install