Fix small typo in the client code

esap_client/shopping_client.py

@@ -17,7 +17,7 @@ logger = logging.getLogger(__name__)
 def is_valid_token(token: str) -> bool:
     """Check that the given token has not expired"""
     try:
-        selfdata = token.split(".")[1]
+        data = token.split(".")[1]
         padded = data + "=" * divmod(len(data), 4)[1]
         payload = json.loads(base64.urlsafe_b64decode(padded))
         return payload["exp"] > int(time.time()) + 10