Exporting portlet with relation field fails

I have several relation fields in a portlet. Exporting portlets then fails because a `RelationValue` is not json serialisable:

```
http://localhost:9152/plone/@@export_portlets
Traceback (innermost last):
Module ZPublisher.Publish, line 138, in publish
Module ZPublisher.mapply, line 77, in mapply
Module ZPublisher.Publish, line 48, in call_object
Module collective.exportimport.export_other, line 477, in __call__
Module json, line 251, in dumps
Module json.encoder, line 209, in encode
Module json.encoder, line 431, in _iterencode
Module json.encoder, line 332, in _iterencode_list
Module json.encoder, line 408, in _iterencode_dict
Module json.encoder, line 408, in _iterencode_dict
Module json.encoder, line 332, in _iterencode_list
Module json.encoder, line 408, in _iterencode_dict
Module json.encoder, line 408, in _iterencode_dict
Module json.encoder, line 442, in _iterencode
Module json.encoder, line 184, in default
TypeError: is not JSON serializable
```

A bit related Is [this comment](https://github.com/collective/collective.exportimport/pull/33#issuecomment-883935512) from Philip where he removes some relations code, although I guess this was only active when exporting content, and not portlets.

The following diff in the portlet export code fixes it for me:

```
$ git diff
diff --git a/src/collective/exportimport/export_other.py b/src/collective/exportimport/export_other.py
index f358a1c..383635a 100644
--- a/src/collective/exportimport/export_other.py
+++ b/src/collective/exportimport/export_other.py
@@ -535,13 +535,18 @@ def export_local_portlets(obj):
settings = IPortletAssignmentSettings(assignment)
if manager_name not in items:
items[manager_name] = []
+ from z3c.relationfield.relation import RelationValue
+ assignment_data = {}
+ for name in schema.names():
+ value = getattr(assignment, name, None)
+ if value and isinstance(value, RelationValue):
+ value = value.to_object.UID()
+ assignment_data[name] = value
+
items[manager_name].append({
'type': portlet_type,
'visible': settings.get('visible', True),
- 'assignment': {
- name: getattr(assignment, name, None)
- for name in schema.names()
- },
+ 'assignment': assignment_data,
})
return items
```

The code needs to be more robust, but those are details.
I am not sure if this is a reasonable place for this fix or if there is a more general place.

Ah, wait, using this works too:

```
json_compatible(getattr(assignment, name, None))
```

At least then you get an export without errors, although my earlier code that returns uuids could be preferable in some cases.

Start in src/collective/exportimport/export_other.py at export_local_portlets and inspect how assignment values are assembled before json.dumps. Compare the reported RelationValue handling with json_compatible; done means exporting portlets containing relation fields completes without a serialization error and preserves the intended relation representation.

Written by the indexing model from the issue text.