Fix memory leaks for Python dicts in C extension

test/memory_leak_deserialize.py

@@ -0,0 +1,17 @@
+# This code reproduces a memory leak we had in the past
+
+import os
+import numpy as np
+import ray
+import ray.worker
+import ray.services as services
+
+worker_path = os.path.join(os.path.dirname(os.path.abspath(__file__)), "test_worker.py")
+services.start_singlenode_cluster(return_drivers=False, num_workers_per_objstore=1, worker_path=worker_path)
+
+d = {"w": np.zeros(1000000)}
+
+obj_capsule, contained_objrefs = ray.lib.serialize_object(ray.worker.global_worker.handle, d)
+
+while True:
+  ray.lib.deserialize_object(ray.worker.global_worker.handle, obj_capsule)