reports: Add RelatedPostings.__repr__() method.

conservancy_beancount/reports/core.py

@@ -195,6 +195,9 @@ class RelatedPostings(Sequence[data.Posting]):
         for value, posts in mapping.items():
             yield value, cls(posts, _can_own=True)
 
+    def __repr__(self) -> str:
+        return f'<{type(self).__name__} {self._postings!r}>'
+
     @overload
     def __getitem__(self: RelatedType, index: int) -> data.Posting: ...