docs: def password_get(username=None):
      """
      Retrieves a password from the keychain based on the environment and
      configuration parameter pair.
  
      If this fails, None is returned.
      """
      password = keyring.get_password('supernova', username)
      if password is None:
          split_username = tuple(username.split(':'))
          msg = ("Couldn't find a credential for {0}:{1}. You need to set one "
                 "with: supernova-keyring -s {0} {1}").format(*split_username)
          raise LookupError(msg)
      else:
          return password.encode('ascii')