Do not serialize mappings with custom __repr__ as dict-s

Fixes #1296

sentry_sdk/serializer.py

@@ -288,7 +288,9 @@ def _serialize_node_impl(
                 else safe_repr(obj)
             )
 
-        elif isinstance(obj, Mapping):
+        elif isinstance(obj, Mapping) and type(obj).__repr__ in (
+            object.__repr__, dict.__repr__
+        ):
             # Create temporary copy here to avoid calling too much code that
             # might mutate our dictionary while we're still iterating over it.
             obj = dict(iteritems(obj))

tests/test_serializer.py

@@ -64,3 +64,13 @@ def test_bytes_serialization_repr(message_normalizer):
 def test_serialize_sets(extra_normalizer):
     result = extra_normalizer({1, 2, 3})
     assert result == [1, 2, 3]
+
+
+def test_serialize_custom_mapping(extra_normalizer):
+    class CustomReprDict(dict):
+        def __repr__(self):
+            return "custom!"
+
+    result = extra_normalizer(CustomReprDict(one=1, two=2))
+    assert result == "custom!"
+