subscriptions create failed due to Object of type 'bytes' is not JSON serializable

When creating subscriptions in queues, zaqar server raise an error: TypeError: Object of type 'bytes' is not JSON serializable

the response_body has bytes value like this, the value of source is bytes type:
{'subscriptions': [{'id': '20fce66f-eed0-4d52-9a57-25fca7c2b349', 'source': b'351465', 'subscriber': b'mailto:<email address hidden>', 'ttl': 3155673600, 'age': 1870, 'options': {'subject': '<email address hidden>'}, 'confirmed': True}], 'links': [{'rel': 'next', 'href': '/v2/queues/351465/subscriptions?marker=20fce66f-eed0-4d52-9a57-25fca7c2b349'}]}

Most projects are update json module to jsonutils of oslo.serialization, consider to update in zaqar:
https://review.opendev.org/q/Update+json+module+to+jsonutils+status:merged

Full trace is:
Failed to send email because a bytes-like object is required, not 'str'.: TypeError: a bytes-like object is required, not 'str'
 Traceback (most recent call last):
   File "/usr/lib/python3.6/site-packages/zaqar/notification/tasks/mailto.py", line 103, in execute
     p.communicate(msg.as_string())
   File "/usr/lib64/python3.6/subprocess.py", line 848, in communicate
     self._stdin_write(input)
   File "/usr/lib64/python3.6/subprocess.py", line 801, in _stdin_write
     self.stdin.write(input)
 zaqar.notification.tasks.mailto TypeError: a bytes-like object is required, not 'str'
 zaqar.notification.tasks.mailto

Object of type 'bytes' is not JSON serializable: TypeError: Object of type 'bytes' is not JSON serializable
 Traceback (most recent call last):
   File "falcon/api.py", line 269, in falcon.api.API.__call__
   File "/usr/lib/python3.6/site-packages/zaqar/common/decorators.py", line 48, in wrapper
     return func(*args, **kwargs)
   File "/usr/lib/python3.6/site-packages/zaqar/transport/acl.py", line 48, in handler
     return func(*args, **kwargs)
   File "/usr/lib/python3.6/site-packages/zaqar/transport/wsgi/v2_0/subscriptions.py", line 171, in on_get
     resp.body = utils.to_json(response_body)
   File "/usr/lib/python3.6/site-packages/zaqar/transport/utils.py", line 63, in to_json
     return json.dumps(obj, ensure_ascii=False)
   File "/usr/lib64/python3.6/json/__init__.py", line 238, in dumps
     **kw).encode(obj)
   File "/usr/lib64/python3.6/json/encoder.py", line 199, in encode
     chunks = self.iterencode(o, _one_shot=True)
   File "/usr/lib64/python3.6/json/encoder.py", line 257, in iterencode
     return _iterencode(o, 0)
   File "/usr/lib64/python3.6/json/encoder.py", line 180, in default
     o.__class__.__name__)
 zaqar.transport.wsgi.driver TypeError: Object of type 'bytes' is not JSON serializable