Ignore construct result for mypy compliance

This commit addresses the mypy issue below by ignoring the construct
result.

```
error: Item "None" of "bytes | str | dict[Any, Any] | Graph | Document | None" has no attribute "serialize" [union-attr]
```

gn3/db/rdf.py

@@ -42,4 +42,4 @@ def sparql_construct_query(query: str, endpoint: str) -> dict:
     sparql = SPARQLWrapper(endpoint)
     sparql.setQuery(query)
     results = sparql.queryAndConvert()
-    return json.loads(results.serialize(format="json-ld"))
+    return json.loads(results.serialize(format="json-ld"))  # type: ignore