Use msgpack instead of pickle in the Heartbeat demo

.gitignore

@@ -33,6 +33,6 @@ variables*.yml
 *ansible/inventory/*
 .env
 examples/heartbeat_demo/*.json
-examples/heartbeat_demo/*.pickle
+examples/heartbeat_demo/*.msgpack
 examples/heartbeat_demo/doctor-files/
 examples/heartbeat_demo/alicia-files/

examples/heartbeat_demo/doctor.py

@@ -2,7 +2,7 @@ import json
 import os
 import sys
 import shutil
-import pickle
+import msgpack
 import maya
 import traceback
 from timeit import default_timer as timer
@@ -82,7 +82,7 @@ doctor.join_policy(label, alices_sig_pubkey)
 # But first we need some encrypted data!
 # Let's read the file produced by the heart monitor and unpack the MessageKits,
 # which are the individual ciphertexts.
-data = pickle.load(open("heart_data.pickle", "rb"))
+data = msgpack.load(open("heart_data.msgpack", "rb"), raw=False)
 message_kits = (UmbralMessageKit.from_bytes(k) for k in data['kits'])
 
 # The doctor also needs to create a view of the Data Source from its public keys
@@ -103,7 +103,7 @@ for message_kit in message_kits:
         )
         end = timer()
 
-        plaintext = pickle.loads(retrieved_plaintexts[0])
+        plaintext = msgpack.loads(retrieved_plaintexts[0], raw=False)
 
         # Now we can get the heart rate and the associated timestamp,
         # generated by the heart rate monitor.

examples/heartbeat_demo/heart_monitor.py

@@ -1,11 +1,11 @@
 import random
 import time
-import pickle
+import msgpack
 
 from nucypher.data_sources import DataSource
 
 
-HEART_DATA_FILENAME = 'heart_data.pickle'
+HEART_DATA_FILENAME = 'heart_data.msgpack'
 
 # Alicia defined a label to categorize all her heart-related data ❤️
 # All DataSources that produce this type of data will use this label.
@@ -37,7 +37,7 @@ def generate_heart_rate_samples(policy_pubkey,
             'timestamp': now,
         }
 
-        plaintext = pickle.dumps(heart_rate_data)
+        plaintext = msgpack.dumps(heart_rate_data, use_bin_type=True)
         message_kit, _signature = data_source.encapsulate_single_message(plaintext)
 
         kit_bytes = message_kit.to_bytes()
@@ -49,6 +49,7 @@ def generate_heart_rate_samples(policy_pubkey,
     }
 
     if save_as_file:
-        pickle.dump(data, open(HEART_DATA_FILENAME, "wb"))
+        with open(HEART_DATA_FILENAME, "wb") as file:
+            msgpack.dump(data, file, use_bin_type=True)
 
     return data