Thanks--it turned out the lack of the universe repository was the issue. Doing:
$ sudo add-apt-repository universe Solved everything. -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1800287 Title: Package python3-mock is not available To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/python-mock/+bug/1800287/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
