[networkx] Type max_flow_min_cost endpoints as nodes (#16559)

Author: Cherith1222

stubs/networkx/networkx/algorithms/flow/mincost.pyi

@@ -17,5 +17,5 @@ def min_cost_flow(
 def cost_of_flow(G: Graph[_Node], flowDict: SupportsGetItem[Incomplete, Incomplete], weight: str = "weight") -> int | float: ...
 @_dispatchable
 def max_flow_min_cost(
-    G: Graph[_Node], s: str, t: str, capacity: str = "capacity", weight: str = "weight"
+    G: Graph[_Node], s: _Node, t: _Node, capacity: str = "capacity", weight: str = "weight"
 ) -> dict[Incomplete, dict[Incomplete, Incomplete]]: ...