Thanks--it turned out the lack of the universe repository was the issue.
Doing:
$ sudo add-apt-repository universe
Solved everything.
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https://bugs.launchpad.net/bugs/1800287
Title:
Public bug reported:
On a server with a fresh install of Ubuntu 18.04 within the past week,
I'm unable install the python3-mock package:
=
$ sudo apt-get install python3-mock
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package python3-mock is
