keyring file-backend reports backtrace with oauth

When bringing the importer back up, I ran into the following:

The authorization page:
 (<ELIDED>)
should be opening in your browser. Use your browser to authorize
this program to access Launchpad on your behalf.
Waiting to hear from Launchpad about your decision...
Traceback (most recent call last):
  File "./bin/usd", line 17, in <module>
    main()
  File "/home/ubuntu/usd-importer/usd/__main__.py", line 172, in main
    args.func(args)
  File "/home/ubuntu/usd-importer/usd/importer.py", line 1230, in main
    lp = launchpad_login_auth()
  File "/home/ubuntu/usd-importer/usd/source_information.py", line 47, in launchpad_login_auth
    version=_lp_api_version)
  File "/usr/lib/python3/dist-packages/launchpadlib/launchpad.py", line 544, in login_with
    credential_save_failed, version)
  File "/usr/lib/python3/dist-packages/launchpadlib/launchpad.py", line 351, in _authorize_token_and_login
    credentials = authorization_engine(credentials, credential_store)
  File "/usr/lib/python3/dist-packages/launchpadlib/credentials.py", line 552, in __call__
    credential_store.save(credentials, self.unique_consumer_id)
  File "/usr/lib/python3/dist-packages/launchpadlib/credentials.py", line 307, in save
    raise e
  File "/usr/lib/python3/dist-packages/launchpadlib/credentials.py", line 302, in save
    self.do_save(credentials, unique_consumer_id)
  File "/usr/lib/python3/dist-packages/launchpadlib/credentials.py", line 384, in do_save
    'launchpadlib', unique_key, serialized)
  File "/usr/lib/python3/dist-packages/keyring/core.py", line 48, in set_password
    _keyring_backend.set_password(service_name, username, password)
  File "/usr/lib/python3/dist-packages/keyring/backends/file.py", line 87, in set_password
    password_encrypted = self.encrypt(password.encode('utf-8'))
AttributeError: 'bytes' object has no attribute 'encode'

cjwatson on IRC suggested this is probably a simple change in launchpadlib:

=== modified file 'src/launchpadlib/credentials.py'
--- src/launchpadlib/credentials.py 2016-08-15 10:32:44 +0000
+++ src/launchpadlib/credentials.py 2017-04-24 16:28:46 +0000
@@ -380,7 +380,7 @@ class KeyringCredentialStore(CredentialS
         # this problem by base 64 encoding the serialized value.
         serialized = self.B64MARKER + b64encode(serialized)
         keyring.set_password(
- 'launchpadlib', unique_key, serialized)
+ 'launchpadlib', unique_key, serialized.decode('utf8'))

     def do_load(self, unique_key):
         """Retrieve credentials from the keyring."""

which ensures unicode is always passed down, but it needs further testing. It does fix the issue in my particular case.