Make sure the session is shutdown on destruction

CHANGELOG.rst

@@ -14,6 +14,7 @@ Bug Fixes
 * Avoid to replace a connection that is supposed to shutdown (PYTHON-772)
 * request_ids may not be returned to the pool (PYTHON-739)
 * Fix murmur3 on big-endian systems (PYTHON-653)
+* Ensure unused connections are closed if a Session is deleted by the GC (PYTHON-774)
 
 Other
 -----

cassandra/cluster.py

@@ -2325,6 +2325,10 @@ class Session(object):
     def __exit__(self, *args):
         self.shutdown()
 
+    def __del__(self):
+        # Ensure all connections are closed, in case the Session object is deleted by the GC
+        self.shutdown()
+
     def add_or_renew_pool(self, host, is_host_addition):
         """
         For internal use only.