Comment # 1 on bug 1096755 from Johannes Meixner

https://bugzilla.redhat.com/show_bug.cgi?id=1528851#c3 tells
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Reported as
https://bugs.launchpad.net/hplip/+bug/1741214
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therein it seems
https://launchpadlibrarian.net/352923466/hplip-jpeg-compression.patch
(long lines may show wrapped here)
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diff -up hplip-3.17.11/scan.py.jpeg-compression hplip-3.17.11/scan.py
--- hplip-3.17.11/scan.py.jpeg-compression    2018-01-08 09:26:42.547072815
+0100
+++ hplip-3.17.11/scan.py    2018-01-08 09:29:39.753604411 +0100
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/python3
 # -*- coding: utf-8 -*-
 #
 # (c) Copyright 2003-2015 HP Development Company, L.P.
@@ -1079,6 +1079,7 @@ try:
             log.info("Saving to file %s" % output)

             try:
+                im = im.convert("RGB")
                 im.save(output)
             except IOError as e:
                 log.error("Error saving file: %s (I/O)" % e)

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is the right final patch.