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
Seems like they moved some things around. This seems to work, as far as getting things, and compiling them. It takes a long time to build.
For the associated designer, look at:
QtDesigner for Raspberry Pi
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/4.19.23/sip-4.19.23.tar.gz
sudo tar xzf sip-4.19.23.tar.gz
cd sip-4.19.23
sudo python3 configure.py --sip-module PyQt5.sip
sudo make
sudo make install
cd /usr/src
sudo wget https://www.riverbankcomputing.com/static/Downloads/PyQt5/5.13.2/PyQt5-5.13.2.tar.gz
sudo tar xzf PyQt5-5.13.2.tar.gz
cd PyQt5-5.13.2
sudo python3 configure.py
sudo make
sudo make install