hplip-install doesn't find the already installed python3-pyqt5 (et al)

I'm using Ubuntu 19.10 with hplip 3.19.11 (which supports ubuntu 19.10).
I have problems already since ubuntu 19.04.

When running hplip-install, I experience several problems.
First, it seems that the installer doesn't use anymore "sudo" by default, but tries to run "su". entering my sudo password doesn't work, so I had to give root a password so that "su" works too next to the "sudo" I usually use.

Second, if I try to run "sudo ./hplip-install", it correctly detects I'm running Ubuntu 19.10, but hplip-install shouldn't be run as root. When I correctly run hplip-install as normal user, it asks whether I'm running Debian, I have to say no, select Ubuntu, then select 19.10.

The real problem comes with the dependencies:
The installer complains that it misses a number of dependencies:

MISSING DEPENDENCIES
--------------------
Following dependencies are not installed. HPLIP will not work if all REQUIRED dependencies are not installed and some of the HPLIP features will not work if OPTIONAL dependencies are not installed.
Package-Name Component Required/Optional
python3-notify2 gui_qt5 OPTIONAL
python3-pyqt5-dbus gui_qt5 OPTIONAL
python3-pyqt5 gui_qt5 REQUIRED
python3-dbus fax REQUIRED
python3-pil scan OPTIONAL
python3-reportlab fax OPTIONAL

hplip-install then quits because it can't install python3-pyqt5.
However, python3-pyqt5 is already installed:

$ sudo apt-get install python3-pyqt5
[sudo] password for janr:
Reading package lists... Done
Building dependency tree
Reading state information... Done
python3-pyqt5 is already the newest version (5.12.3+dfsg-2).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

Similarly,
python3-dbus is already the newest version (1.2.12-1).

So I don't understand why these packages are reported as missing when they are present.

On the other hand,
$ sudo apt-get install python3-pyqt5-dbus
[sudo] password for janr:
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package python3-pyqt5-dbus

So there is one package which is really missing, but this package doesn't exist in ubuntu 19.10.

In conclusion,
* the installer doesn't detect the packages which are already installed,
* the installer wants to install *optional* packages which don't exist.
* I couldn't find how to let the installer continue installation without the optional dependencies present.

Jan