It looks like this API in particular expects all requests to have a User-Agent header, and we don't set one by default. Setting any user agent appears to work; the issue isn't with the `Accept` header.

You can reproduce with curl with the following command:

    curl -v 'https://oneom.tk/data/config' -H 'Accept: application/json' -H 'User-Agent:'

That'll nullify the default curl user agent and should produce the same results you were seeing with urllib3.