fix: use is_open attribute to denote session availability

Author: varun-edachali-dbx

src/databricks/sql/session.py

@@ -136,7 +136,7 @@ def get_session_id_hex(self):
     def close(self) -> None:
         """Close the underlying session."""
         logger.info(f"Closing session {self.get_session_id_hex()}")
-        if not self.open:
+        if not self.is_open:
             logger.debug("Session appears to have been closed already")
             return