Please check  FS#39587  - can you check if hp-config_usb_printer script fails for you with python2 ?

I do not have an exact idea what you mean, sorry. With hplip downgraded I get:
hp-config_usb_printer -g 002:009
hp-config_usb_printer[1395]: debug: param=002:009
hp-config_usb_printer[1395]: debug: Trying USB with bus=002 dev=009...
hp-config_usb_printer[1395]: debug: Found: hp:/usb/DeskJet_840C?serial=CN0B11B0V9KV
hp-config_usb_printer[1395]: debug: hp:/usb/DeskJet_840C?serial=CN0B11B0V9KV: back_end:hp is_hp:True bus:usb model:DeskJet_840C serial:CN0B11B0V9KV dev_file: host: zc: port:1
hp-config_usb_printer[1395]: debug: Cache miss: deskjet_840c
hp-config_usb_printer[1395]: debug: Reading file: /usr/share/hplip/data/models/models.dat
hp-config_usb_printer[1395]: debug: Searching for section [deskjet_840c] in file /usr/share/hplip/data/models/models.dat
hp-config_usb_printer[1395]: debug: Found section [deskjet_840c] in file /usr/share/hplip/data/models/models.dat
hp-config_usb_printer[1395]: debug:
Setting up device: hp:/usb/DeskJet_840C?serial=CN0B11B0V9KV

hp-config_usb_printer[1395]: debug: hp:/usb/DeskJet_840C?serial=CN0B11B0V9KV: back_end:hp is_hp:True bus:usb model:DeskJet_840C serial:CN0B11B0V9KV dev_file: host: zc: port:1
hp-config_usb_printer[1395]: debug: Done.

How's 3.14.4 doing?

If you check the mentioned old bug report you see the change was required to make it actually use python2 and not python 3.x - I wonder
why this now fails for you. It would be nice if you can investigate further what may break it for you. So please try to call the commands from
command line to see where it fails.

It is slightly besides the point but... as a fellow ancient-deskjet user, I say hplip break printing for me on quite a few of the recent upgrades. Clearly, you only notice when you actually need to use the printer, which for me tends to be at very different times than when I have the time to babysit my computer through yet another round of up- and/or downgrades. HPLIP is moreover ludicrously slow for me, in the sense of taking up to 30 seconds to start a job, My ancient deskjet is connected to an ancient-nas -- but with the below method, I get an immediate start...

Our printers are no doubt too old to be part of any testing-farm at HP and it seems to show. I have simply removed HPLIP. You can get your printer working, and perhaps more optimal than through HPLIP, by simply manually downloading a PPD from:

https://www.openprinting.org/printer/HP/HP-DeskJet_840C

and using THAT as a driver. I myself use the generic "pcl3" driver, available from that page for the 840C as:

https://www.openprinting.org/ppd-o-matic.php?driver=pcl3&printer=HP-DeskJet_840C&show=0

You use it by, in the CUPS webinterface -> Printers -> <printer< -> Maintenance -> Modify Printer, selecting the downloaded PPD. If, although I don't expect it, PCL3 doesn't give you more and/or more printer-specific options than your current HPLIP driver, you can of course try a few other drivers from that page as well.

But anyways -- just say no to HPLIP...

hplip-3.14.4-1 seems working well, test page prints.