Provide api version as an enum instead of hardcoded string

Author: samwelkanda

msgraphcore/graph_session.py

@@ -1,6 +1,8 @@
 """
 Graph Session
 """
+from enum import Enum
+
 from requests import Session
 
 from msgraphcore.constants import BASE_URL, SDK_VERSION
@@ -11,6 +13,11 @@
 from msgraphcore.middleware.retry_middleware import RetryMiddleware
 
 
+class APIVersion(str, Enum):
+    beta = 'beta'
+    v1 = 'v1.0'
+
+
 class GraphSession(Session):
     """Extends Session with Graph functionality
 
@@ -22,7 +29,7 @@ def __init__(
         scopes: [str] = ['.default'],
         retry_config: dict = {},
         middleware: list = [],
-        api_version: str = 'v1.0'
+        api_version: APIVersion = APIVersion.v1,
     ):
         super().__init__()
         self._append_sdk_version()