thumbor: Cannot start thumbor (config security key was loaded as binary string not str)

Expected behaviour

Run thumbor successfully

Actual behaviour

Traceback (most recent call last):
  File "~/venv/bin/thumbor", line 11, in <module>
    sys.exit(main())
  File "~/venv/lib/python3.6/site-packages/thumbor/server.py", line 147, in main
    validate_config(config, server_parameters)
  File "~/venv/lib/python3.6/site-packages/thumbor/server.py", line 86, in validate_config
    'Please provide one using the conf file or a security key file.')
RuntimeError: No security key was found for this instance of thumbor. Please provide one using the conf file or a security key file.

Operating system

OSX 10.12.6

Your thumbor.conf

Original config

Debugging

I dump the server_parameters.security_key to check, the server_parameters.security_key is load as a binary string, so not passed the if not isinstance(server_parameters.security_key, basestring): condition, is that something I misunderstood?

Fail if statement code: https://github.com/thumbor/thumbor/blob/master/thumbor/server.py#L80