Loosen validation of `TargetIDMixin`

It now requires one of the target attributes
to be set, not just `target_id`. This fixes
follows over the Diaspora protocol which broke
with stricter send validation added in 0.19.0.

CHANGELOG.md

@@ -19,6 +19,10 @@
   
 * Ensure Pixelfed, Kroeg and Kibou instances that emulate the Mastodon API don't get identified as Mastodon instances.
 
+* Loosen validation of `TargetIDMixin`, it now requires one of the target attributes
+  to be set, not just `target_id`. This fixes follows over the Diaspora protocol which
+  broke with stricter send validation added in 0.19.0. 
+
 ## [0.19.0] - 2019-12-15
 
 ### Added

federation/entities/mixins.py

@@ -148,9 +148,11 @@ class TargetIDMixin(BaseEntity):
     target_handle = ""
     target_guid = ""
 
-    def __init__(self, *args, **kwargs):
-        super().__init__(*args, **kwargs)
-        self._required += ["target_id"]
+    def validate(self, *args, **kwargs) -> None:
+        super().validate(*args, **kwargs)
+        # Ensure one of the target attributes is filled at least
+        if not self.target_id and not self.target_handle and not self.target_guid:
+            raise ValueError("Must give one of the target attributes for TargetIDMixin.")
 
 
 class RootTargetIDMixin(BaseEntity):