digraph.py: add type for cyclic imports as a forward reference

Signed-off-by: Berin Aniesh <berinaniesh@gmail.com>

lib/portage/util/digraph.py

@@ -80,12 +80,9 @@ def remove(self, node: "SetArg"):
         del self.nodes[node]
         self.order.remove(node)
 
-    def update(self, other):
+    def update(self, other: "digraph"):
         """
         Add all nodes and edges from another digraph instance.
-
-        other: another digraph instance,
-        can't be type annotated according to PEP 484 due to cyclic imports
         """
         for node in other.order:
             children, parents, node = other.nodes[node]