Change use of Protocol from typing instead of typing_extension

The graph module uses Protocols, which is natively supported since Python 3.8 (the required minimum version for DoWhy). To support Protocols in earlier Python versions, an extension provided Protocol support. However, this extension can causes compatibility issues with newer versions of other packages.

DoWhy now uses the Protocol implementation that is available since Python 3.8.

Signed-off-by: Patrick Bloebaum <bloebp@amazon.com>

dowhy/graph.py

@@ -5,11 +5,10 @@
 """
 
 from abc import abstractmethod
-from typing import Any, List
+from typing import Any, List, Protocol
 
 import networkx as nx
 from networkx.algorithms.dag import has_cycle
-from typing_extensions import Protocol
 
 
 class HasNodes(Protocol):