Hm, I am not sure if I understand the workflow here.
Without a config file or with a section or key missing in the config file, isrcsubmit should do what isrcsubmit does now (as defined per the constants defined at the top of the script and the defaults for the flags).

I don't fully understand why we want to do the intermediate step of creating a "default config file".
There is no config file written or anything.

I do understand that `config.get("musicbrainz", "user")` raises a `NoSectionError` so this is possibly a workaround? In that case we should use `config.has_option()` instead to test if an option is set in the config file at all and only use it to deviate from the defaults when it is.

So unless I overlook something obvious, we don't need this `fill_config_with_default_values()` at all.