Made longview_subscription immutable in Account Settings (linode#581)

Author: ezilber-akamai

linode_api4/objects/account.py

@@ -194,7 +194,7 @@ class AccountSettings(Base):
         "network_helper": Property(mutable=True),
         "managed": Property(),
         "longview_subscription": Property(
-            slug_relationship=LongviewSubscription
+            slug_relationship=LongviewSubscription, mutable=False
         ),
         "object_storage": Property(),
         "backups_enabled": Property(mutable=True),