That "mainline" is only installer tool:

https://github.com/cappelikan/mainline

You don't need to use that mainline installer.

You can download mainline kernel .deb files directly from Ubuntu:

https://kernel.ubuntu.com/mainline/

https://kernel.ubuntu.com/mainline/v7.3.1/

I found here Canonical commenting about status of new kernels:

https://news.ycombinator.com/item?id=50348688