ENH: Added graph sort to conda dependency resolution
TEST: added toposort tests
FIX: for python3
ENH: removed obsolete code

@asmeurer
ENH: updated cycle detection to pop sorted key

`pop_key` will pop an item from the graph that has the fewest dependencies
In the case of a tie, the winners will be sorted alphabetically.

cc @mattpap @asmeurer
FIX: python3
Merge pull request #1009 from conda/feature/topo-sort

Added graph sort to conda dependency resolution