Use dataclasses to define activitypub (de)serialization (#177)

* Use dataclasses to define activitypub (de)serialization

fedireads/broadcast.py

@@ -4,6 +4,7 @@ from django.utils.http import http_date
 import requests
 
 from fedireads import models
+from fedireads.activitypub import ActivityEncoder
 from fedireads.tasks import app
 from fedireads.signatures import make_signature, make_digest
 
@@ -38,7 +39,11 @@ def broadcast(sender, activity, software=None, \
     # TODO: other kinds of privacy
     if privacy == 'public':
         recipients += get_public_recipients(sender, software=software)
-    broadcast_task.delay(sender.id, activity, recipients)
+    broadcast_task.delay(
+        sender.id,
+        json.dumps(activity, cls=ActivityEncoder),
+        recipients
+    )
 
 
 @app.task