How to get back modified fields from a bulk update?

[bra-fsn]

Elasticsearch version (bin/elasticsearch --version): 7.7.0

elasticsearch-py version (elasticsearch.__versionstr__): 7.5.1

Description of the problem including expected versus actual behavior:
With previous elasticsearch version, this code snippet has worked:

bulkops.append({'_op_type': 'update',
                                '_index': elastic_index,
                                '_type': elastic_doctype,
                                '_id': oid,
                                'retry_on_conflict': 8,
                                'fields': '_source',
                                'doc': {'field_name': 'new_value'}})
res = helpers.parallel_bulk(self.es, bulkops,
                                            raise_on_exception=False,
                                            raise_on_error=False)

But after upgrading elasticsearch to 7.x, this fails with:

'error': "RequestError(400, 'x_content_parse_exception', '[1:2] "
              "[UpdateRequest] unknown field [fields]')",
'exception': RequestError(400, 'x_content_parse_exception', {'error': {'root_cause': [{'type': 'x_content_parse_exception', 'reason': '[1:2] [UpdateRequest] unknown field [fields]'}], 'type': 'x_content_parse_exception', 'reason': '[1:2] [UpdateRequest] unknown field [fields]'}, 'status': 400}),

What I'm trying to accomplish here is to get back the modified document with the bulk updates.

With curl, I can do the following:
curl -s -H "Content-Type: application/x-ndjson" -XPOST 'localhost:9200/_bulk?pretty&_source=true' --data-binary "@requests"

So I think the question is: how to specify query parameters with the helpers.bulk API? As it can be seen from the doc and the above curl call, elasticsearch itself supports this.

Steps to reproduce:
Execute the above code on a 7.x elasticsearch.

[sethmlarson]

This was implemented in 7.10 via #1387