Add graphviz as a dependency and update import statements

Author: MartinEBravo

src/agents/extensions/visualization.py

@@ -1,6 +1,6 @@
 from typing import Optional
 
-import graphviz
+import graphviz  # type: ignore
 
 from agents import Agent
 from agents.handoffs import Handoff

tests/test_visualization.py

@@ -1,6 +1,6 @@
 from unittest.mock import Mock
 
-import graphviz
+import graphviz  # type: ignore
 import pytest
 
 from agents import Agent