Attaching some of my 2013 work on this.  Here are some API notes:

* spell-out the name topological_sort()

* allow input as ordered pairs like the Unix tsort command
  https://en.wikipedia.org/wiki/Tsort#Examples

* allow more convenient input as dependency sequences (like graphviz):
     [['a', 'b', 'c', 'x], ['b', 'd', 'e', 'y']]
  is short for and equivalent to:
     [(a,b), (b,c), (c,x), (b,d), (d, e), (e, y)]

* return both the sorted sequence and cycles
  (both are individually useful and the latter
   is helpful in debugging in only the former is wanted)

* desire to match the C3 MRO computation


* would like the ordering to be stable and deterministic
  (this precludes picking arbitrary elements from sets).