* Fix performance issue with large datasets.
* Use f-strings instead of str.format().
* Fix issue #2: Allow non-orderable items when a
CircularDependencyError is raised
* Completely change to pyproject.toml. Deleted setup.cfg.
* Drop support for python 2.7.
* No functional changes.
OBS-URL: https://build.opensuse.org/package/show/devel:languages:python/python-toposort?expand=0&rev=4