Add property expiry in gspread client

In gspread client add a new property expiry which returns the expiry
datetime object from the used credentials.

The expiry only exists for OAuth credentials otherwise it returns None.

closes #1169

Signed-off-by: Alexandre Lavigne <[email protected]>

gspread/client.py

@@ -6,6 +6,7 @@
 
 """
 
+from datetime import datetime
 from http import HTTPStatus
 from typing import Any, Dict, List, Optional, Tuple, Union
 
@@ -39,6 +40,18 @@ def __init__(
     ) -> None:
         self.http_client = http_client(auth, session)
 
+    @property
+    def expiry(self) -> Optional[datetime]:
+        """Returns the expiry date of the curenlty loaded credentials
+
+        :returns: (optional) datetime the expiry date time object.
+
+        .. note::
+
+           It only applies to gspread client created using oauth
+        """
+        return self.http_client.auth.expiry
+
     def set_timeout(
         self, timeout: Optional[Union[float, Tuple[float, float]]] = None
     ) -> None: