Thanks, I forgot that the older repos also received the pg-10 update.
Unfortunately but no luck with Xenial either, slightly different
but similar conflicts.
My main motivation for updating to 10.3 was to be able to load data
dumped from a 10.3 database. pg_restore complained about "unsupported
version (1.13) in file header". However I just discovered I can load
a plain sql dump from that database so panic mode is over :-) and I
can wait until I can upgrade my machine to ubuntu-18.04.
It still seems to me that the best advice for using Postgresql on
Ubuntu is to use the Ubuntu version of Postgresql if you don't need
the latest version; if you do need latest version, use the Pgdg
version but only with a LTS version of Ubuntu.
If you need the latest version of both Ubuntu and Postgresql, you
may be out of luck.
Or you compile it?
