As you can see it is gzip
$ curl -I api.stackexchange.com/2.1/sites HTTP/1.1 400 Bad Request Cache-Control: private Content-Length: 95 Content-Type: application/json; charset=utf-8 Content-Encoding: gzip
So
$ curl -s --compressed api.stackexchange.com/2.1/sites {"items":[{"site_type":"main_site","name":"Stack Overflow","logo_url":"http://cd n.sstatic.net/stackoverflow/img/logo.png","api_site_parameter":"stackoverflow"," site_url":"http://stackoverflow.com","audience":"professional and enthusiast pro [...]
Tricky Tricky