Remove dependency on `python-graph-core`.

The maintainer has removed 1.6.2 from the internet, and it's not cached on
PyPI. I've replaced the one thing sparkplug uses -- dependency analysis via
topological sort -- with an implementation of a known algorithm.
Give internal config objects better debugging reprs.
Merge pull request #6 from ojacobson/no-python-graph-core

Remove dependency on `python-graph-core`.