Nope, tried all of the suggestions.

I have both python2.7 and python3.6 installed, and set python with both "alternatives", including running (e.g. for python 3.6):
export PYTHON=/usr/bin/python3
export PYTHONINCLUDEDIR=/usr/include/python3.6
mkdir hplip-3.1.19
cd hplip-3.1.19
sh ~/Downloads/hplip-3.19.1.run --tar vxf
./configure --with-hpppddir=/usr/share/ppd/HP --libdir=/usr/lib/x86_64-linux-gnu --prefix=/usr --enable-network-build --enable-scan-build --enable-fax-build --enable-dbus-build --enable-qt4 --disable-qt5 --disable-class-driver --enable-doc-build --disable-policykit --disable-libusb01_build --disable-udev_sysfs_rules --enable-hpcups-install --disable-hpijs-install --disable-foomatic-ppd-install --disable-foomatic-drv-install --disable-cups-ppd-install --enable-cups-drv-install --enable-apparmor_build
make
sudo make install
rm -rf ~/.hplip
hp-setup

... same result.

I installed manually the plugin:
mkdir hplip-3.19.1-plugin
cd hplip-3.19.1-plugin
sh ~/Downloads/hplip-3.19.1-plugin.run --tar vxf
sudo python3 installPlugin.py
hp-setup

... similar result, specifies binary plugin required, only now it does not offer the download of the binaries.

Although in the console I now see (or maybe oice for the 1st time):
$ hp-setup

HP Linux Imaging and Printing System (ver. 3.19.1)
Printer/Fax Setup Utility ver. 9.0

Copyright (c) 2001-15 HP Development Company, LP
This software comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to distribute it
under certain conditions. See COPYING file for more details.

Searching... (bus=net, timeout=5, ttl=4, search=(None) desc=0, method=slp)
Segmentation fault
error:  The device you are trying to setup requires a binary plug-in. Some functionalities may not work as expected without plug-ins.  Please run 'hp-plugin' as normal user to install plug-ins.   Visit  http://hplipopensource.com  for more infomation. 
Segmentation fault
error:  The device you are trying to setup requires a binary plug-in. Some functionalities may not work as expected without plug-ins.  Please run 'hp-plugin' as normal user to install plug-ins.   Visit  http://hplipopensource.com  for more infomation. 

The segfault comes after I highlight my printer from the discovered devices and select "Next".

Files still exist in the plugin directory:
$ ls -l /usr/share/hplip/prnt/plugins/
total 128
lrwxrwxrwx 1 root root    45 Mar  6 17:01 hbpl1.so -> /usr/share/hplip/prnt/plugins/hbpl1-x86_64.so
-rwxr-xr-x 1 root root 49528 Mar  6 17:01 hbpl1-x86_64.so
lrwxrwxrwx 1 root root    42 Mar  6 17:01 lj.so -> /usr/share/hplip/prnt/plugins/lj-x86_64.so
-rwxr-xr-x 1 root root 74496 Mar  6 17:01 lj-x86_64.so


Oddly, if I run the /usr/bin/hp-plugin or its target /usr/share/hplip/plugin.py it'll die with:
...
Plug-in version: 3.19.1
Installed HPLIP version: 3.19.1
Number of files to install: 64

Plugin installation failed
error: Python gobject/dbus may be not installed
error: Plug-in install failed.

Done.


But python2 or python3 /usr/share/hplip/plugin.py will not fail:
$ python /usr/share/hplip/plugin.py 

HP Linux Imaging and Printing System (ver. 3.19.1)
Plugin Download and Install Utility ver. 2.1

Copyright (c) 2001-15 HP Development Company, LP
This software comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to distribute it
under certain conditions. See COPYING file for more details.


HP Linux Imaging and Printing System (ver. 3.19.1)
Plugin Download and Install Utility ver. 2.1

Copyright (c) 2001-15 HP Development Company, LP
This software comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to distribute it
under certain conditions. See COPYING file for more details.

Downloading plug-in from: file:///home/alexs/Downloads/hplip-3.19.1-plugin.run
1100%Receiving digital keys: /usr/bin/gpg --homedir /home/alexs/.hplip/.gnupg --no-permission-warning --keyserver pgp.mit.edu --recv-keys 0x4ABA2F66DBD5A95894910E0673D770CDA59047B9
Creating directory plugin_tmp
Verifying archive integrity... All good.
Uncompressing HPLIP 3.19.1 Plugin Self Extracting Archive..............................................................

HP Linux Imaging and Printing System (ver. 3.19.1)
Plugin Installer ver. 3.0

Copyright (c) 2001-15 HP Development Company, LP
This software comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to distribute it
under certain conditions. See COPYING file for more details.

Plug-in version: 3.19.1
Installed HPLIP version: 3.19.1
Number of files to install: 64

 
Done.
 Plug-in installation successful 

Done.
$


This makes me think something odd is going on with the shebang 
  /usr/bin/env python
which incidentally gives:
/usr/bin/env python
Python 3.6.7 (default, Oct 22 2018, 11:32:17) 
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

which is exactly the same as /usr/bin/python

Well, thats another day wasted trying to print a single page, but hopefully a little closer as running through env, which I assume hplip's pythons scripts are doing, seems to be messing this up.

Any additional suggestions greatfully appreciated.