improve hint typing

Graph/graph/graph_2.py

@@ -13,7 +13,7 @@ class NDGraph:
      below I implemented the 2nd version.
     """
 
-    def __init__(self, nodes):
+    def __init__(self, nodes: Sequence['Node']) -> None:
         self.nodes: Set['Node'] = {n for n in nodes}
         self.vertices: Dict['Node': Set['Node']] = {n: set() for n in self.nodes}
 
@@ -80,10 +80,10 @@ class Node:
 
     _id = itertools.count(0)
 
-    def __init__(self):
+    def __init__(self) -> None:
         self.id: int = next(self._id)
 
-    def __hash__(self):
+    def __hash__(self) -> int:
         # to be usable in set an object must be hashable
         # so we need to implement __hash__
         # which must returm an int uniq per object