🎨 improve hint typing

Graph/graph/graph_1.py

 import itertools
 
-from typing import Iterator, Optional
+from typing import Iterator, Set, Optional
 
 
 class NDGraph:
@@ -14,7 +14,7 @@ class NDGraph:
     """
 
     def __init__(self):
-        self.nodes = set()
+        self.nodes: Set['Node'] = set()
 
     def add_node(self, node: 'Node') -> None:
         self.nodes.add(node)
@@ -29,7 +29,7 @@ class Node:
 
     def __init__(self):
         self.id: int = next(self._id)
-        self.neighbors = set()
+        self.neighbors: Set['Node'] = set()
 
     def __hash__(self):
         # to be usable in set an object must be hashable

Graph/graph/graph_2.py

 import itertools
 
-from typing import Iterator, Set, Sequence
+from typing import Iterator, Dict, Set, Sequence
 
 
 class NDGraph:
@@ -14,8 +14,8 @@ class NDGraph:
     """
 
     def __init__(self, nodes):
-        self.nodes = {n for n in nodes}
-        self.vertices = {n: set() for n in self.nodes}
+        self.nodes: Set['Node'] = {n for n in nodes}
+        self.vertices: Dict['Node': Set['Node']] = {n: set() for n in self.nodes}
 
 
     def add_node(self, node: 'Node') -> None: