Rename credits section to attributions.

Add gtk-osx to license section.

Previously, .gitignore files were misused for this purpose. In part
they also served the purpose of supporting in-tree builds without
polluting the git status, but that idea was abandoned long ago.

While it's no longer short and sweet, it's a lot more self-explanatory
now.

Use the system's Python to bootstrap.

Job name isn't interesting and IDs are in the URLs.