Topological Sort in Python with Tests

Topological Sort holds 21 problems (13 Medium, 8 Hard). Generate any of them into the current directory with lcpy gen -n <number>.

#	Problem	Difficulty	Solution
207	Course Schedule	Medium	solution.py
210	Course Schedule II	Medium	solution.py
269	Alien Dictionary	Hard	solution.py
310	Minimum Height Trees	Medium	solution.py
329	Longest Increasing Path in a Matrix	Hard	solution.py
444	Sequence Reconstruction	Medium	solution.py
631	Design Excel Sum Formula	Hard	solution.py
802	Find Eventual Safe States	Medium	solution.py
851	Loud and Rich	Medium	solution.py
913	Cat and Mouse	Hard	solution.py
1059	All Paths from Source Lead to Destination	Medium	solution.py
1136	Parallel Courses	Medium	solution.py
1245	Tree Diameter	Medium	solution.py
1462	Course Schedule IV	Medium	solution.py
1857	Largest Color Value in a Directed Graph	Hard	solution.py
1976	Number of Ways to Arrive at Destination	Medium	solution.py
2050	Parallel Courses III	Hard	solution.py
2115	Find All Possible Recipes from Given Supplies	Medium	solution.py
2127	Maximum Employees to Be Invited to a Meeting	Hard	solution.py
2392	Build a Matrix With Conditions	Hard	solution.py
3481	Apply Substitutions	Medium	solution.py