Add scikit-build to the list of non-PyPA projects of interest

[ncoghlan]

As enscons is to Scons, scikit-build is to CMake (they even release a pip installable CMake build to make that possible).

Docs: https://scikit-build.readthedocs.io/

(Discovered while researching my options for making the libsolv Python bindings pip installable)

[nathanielcompton]

@di Saw that you took over packaging, and tagging you here. We should be able to close this issue as #530 resolved it.

As I get more free time, I'll try to find more close-able issues in this repo 👍

[pradyunsg]

Thanks @nathanielcompton!