Use pathlib instead of os.path

pathlib has been around for a while (since Python 3.4) and we have been encourage its usage in the examples and many places of the pyside-setup repository, but still there are many usages around the build_scripts and other tools that rely on os.path.

Sadly, pathlib cannot replace fully os.path, particularly there is a function os.path.relpath that cannot be migrated to Path.relative_to due to the inability of generating relative paths when there is no direct parent-child relationship between the paths, but os.path.relpath is capable of generating things like ../../something.