Graphs/kruskal: adding doctest & type hints (TheAlgorithms#3101)

* graphs/kruskal: add doctest & type hints

this is a child of a previous PR TheAlgorithms#2443

its ancestor is TheAlgorithms#2128

* updating DIRECTORY.md

* graphs/kruskal: fix max-line-length violation

* fixup! Format Python code with psf/black push

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

graphs/minimum_spanning_tree_kruskal.py

@@ -1,4 +1,18 @@
-def kruskal(num_nodes, num_edges, edges):
+from typing import List, Tuple
+
+
+def kruskal(num_nodes: int, num_edges: int, edges: List[Tuple[int, int, int]]) -> int:
+    """
+    >>> kruskal(4, 3, [(0, 1, 3), (1, 2, 5), (2, 3, 1)])
+    [(2, 3, 1), (0, 1, 3), (1, 2, 5)]
+
+    >>> kruskal(4, 5, [(0, 1, 3), (1, 2, 5), (2, 3, 1), (0, 2, 1), (0, 3, 2)])
+    [(2, 3, 1), (0, 2, 1), (0, 1, 3)]
+
+    >>> kruskal(4, 6, [(0, 1, 3), (1, 2, 5), (2, 3, 1), (0, 2, 1), (0, 3, 2),
+    ... (2, 1, 1)])
+    [(2, 3, 1), (0, 2, 1), (2, 1, 1)]
+    """
     edges = sorted(edges, key=lambda edge: edge[2])
 
     parent = list(range(num_nodes))