This fixes a bunch of minor pylint warnings:
* Drop unused imports.
* Fix "inline-JSON" formatting.
* Fix space before/after brackets.
* Use `_` for unused variables.
* Break overlong lines.
* Mark unittest as `no-self-use` if applicable.
* Drop spurious newline at end of file.
When using rpm-ostree compose, a Treefile[1] controls various
aspects of its behaviour. Since rpm-ostree will, at least in
the beginning, be used to post-process and committing the tree
add a helper class to ease the creation of correct Treefiles.
The docstring of the Treefile contains the information in which
phases ('install', 'postprocess', 'commit') the option is used,
as of rpm-ostree commit 1cf0d557ae8059e689b1fed670022727e9842288
Add basic checks for the ostree.Treefile helper. Some of the
tests require rpm-ostree to be installed.
[1] https://rpm-ostree.readthedocs.io/en/stable/manual/treefile/
