Getting cluster config fails with encoding error (base64)

When I run try to retrieve the configuration of a Kubernetes cluster spawned by Magnum I receive a TypeError.

''openstack --debug coe cluster config <cluster>'' returns the following traceback:

Traceback (most recent call last):
  File "/home/yannic/Envs/os-dev/local/lib/python3.6/site-packages/cliff/app.py", line 401, in run_subcommand
 result = cmd.run(parsed_args)
  File "/home/yannic/Envs/os-dev/local/lib/python3.6/site-packages/osc_lib/command/command.py", line 41, in run
 return super(Command, self).run(parsed_args)
  File "/home/yannic/Envs/os-dev/local/lib/python3.6/site-packages/cliff/command.py", line 184, in run
 return_code = self.take_action(parsed_args) or 0
  File "/home/yannic/Envs/os-dev/local/lib/python3.6/site-packages/magnumclient/osc/v1/clusters.py", line 353, in take_action
 certs=tls))
  File "/home/yannic/Envs/os-dev/local/lib/python3.6/site-packages/magnumclient/common/utils.py", line 167, in config_cluster
 force, certs)
  File "/home/yannic/Envs/os-dev/local/lib/python3.6/site-packages/magnumclient/common/utils.py", line 217, in _config_cluster_kubernetes
 'key': base64.b64encode(certs['key']),
  File "/home/yannic/Envs/os-dev/lib/python3.6/base64.py", line 58, in b64encode
 encoded = binascii.b2a_base64(s, newline=False)
TypeError: a bytes-like object is required, not 'str'

I figure that replacing the offending statements (e.g. 'key': base64.b64encode(certs['key'])) with the corresponding oslo functionality (oslo_serialization.base64.encode_as_text) resolves the problem.

* Platform: Ubuntu 18.10
* Python 3.6.7
* python-magnumclient: 2.11 (reproducible with upstream master)