This is just something the site does. It appears to be part of some kind of anti-DDoS system. Why it returns 503 is perplexing, but it is definitely the site itself.
I tried the curl command Joe has above, and this is the response I get back:
HTTP/1.1 503 Service Temporarily Unavailable
Date: Mon, 08 Dec 2014 09:47:41 GMT
Content-Type: text/html; charset=UTF-8
Transfer-Encoding: chunked
Connection: keep-alive
Set-Cookie: __cfduid=d32f001037fafc1363bf86d29be0baf921418032061; expires=Tue, 08-Dec-15 09:47:41 GMT; path=/; domain=.gametracker.com; HttpOnly
X-Frame-Options: SAMEORIGIN
Cache-Control: no-cache
Server: cloudflare-nginx
CF-RAY: 19580b02d7c70f21-IAD
<!DOCTYPE HTML>
<html lang="en-US">
<head>
<meta charset="UTF-8" />
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<meta http-equiv="X-UA-Compatible" content="IE=Edge,chrome=1" />
<meta name="robots" content="noindex, nofollow" />
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1" />
<title>Just a moment...</title>
<style type="text/css">
html, body {width: 100%; height: 100%; margin: 0; padding: 0;}
body {background-color: #ffffff; font-family: Helvetica, Arial, sans-serif; font-size: 100%;}
h1 {font-size: 1.5em; color: #404040; text-align: center;}
p {font-size: 1em; color: #404040; text-align: center; margin: 10px 0 0 0;}
#spinner {margin: 0 auto 30px auto; display: block;}
.attribution {margin-top: 20px;}
</style>
<script type="text/javascript">
//<![CDATA[
(function(){
var a = function() {try{return !!window.addEventListener} catch(e) {return !1} },
b = function(b, c) {a() ? document.addEventListener("DOMContentLoaded", b, c) : document.attachEvent("onreadystatechange", b)};
b(function(){
var a = document.getElementById('cf-content');a.style.display = 'block';
setTimeout(function(){
var t,r,a,f, sdDUenl={"xRvHG":+((!+[]+!![]+[])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]))};
t = document.createElement('div');
t.innerHTML="<a href='/'>x</a>";
t = t.firstChild.href;r = t.match(/https?:\/\//)[0];
t = t.substr(r.length); t = t.substr(0,t.length-1);
a = document.getElementById('jschl-answer');
f = document.getElementById('challenge-form');
;sdDUenl.xRvHG*=+((!+[]+!![]+!![]+[])+(!+[]+!![]+!![]+!![]));sdDUenl.xRvHG-=+((!+[]+!![]+[])+(!+[]+!![]+!![]+!![]+!![]));sdDUenl.xRvHG+=+((!+[]+!![]+!![]+[])+(!+[]+!![]+!![]+!![]));sdDUenl.xRvHG*=+((!+[]+!![]+!![]+!![]+[])+(!+[]+!![]+!![]+!![]+!![]+!![]));sdDUenl.xRvHG-=+((!+[]+!![]+!![]+!![]+!![]+[])+(+[]));sdDUenl.xRvHG-=+((!+[]+!![]+[])+(!+[]+!![]+!![]+!![]+!![]));sdDUenl.xRvHG*=+((!+[]+!![]+[])+(!+[]+!![]+!![]+!![]));sdDUenl.xRvHG-=+((!+[]+!![]+!![]+!![]+[])+(!+[]+!![]+!![]));sdDUenl.xRvHG*=+((+!![]+[])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]));sdDUenl.xRvHG+=+((!+[]+!![]+!![]+!![]+[])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]));a.value = parseInt(sdDUenl.xRvHG, 10) + t.length;
f.submit();
}, 5850);
}, false);
})();
//]]>
</script>
</head>
<body>
<table width="100%" height="100%" cellpadding="20">
<tr>
<td align="center" valign="middle">
<div class="cf-browser-verification cf-im-under-attack">
<noscript><h1 data-translate="turn_on_js" style="color:#bd2426;">Please turn JavaScript on and reload the page.</h1></noscript>
<div id="cf-content" style="display:none">
<img id="spinner" src="/cdn-cgi/images/spinner-2013.gif" />
<h1><span data-translate="checking_browser">Checking your browser before accessing</span> gametracker.com.</h1>
<p data-translate="process_is_automatic">This process is automatic. Your browser will redirect to your requested content shortly.</p>
<p data-translate="allow_5_secs">Please allow up to 5 seconds…</p>
</div>
<form id="challenge-form" action="/cdn-cgi/l/chk_jschl" method="get">
<input type="hidden" name="jschl_vc" value="3cecd7cab5d69708a3b1081e462824d0"/>
<input type="hidden" id="jschl-answer" name="jschl_answer"/>
</form>
</div>
<div class="attribution"><a href="http://www.cloudflare.com/" target="_blank" style="font-size: 12px;">DDoS protection by CloudFlare</a></div>
</td>
</tr>
</table>
</body>
</html>
Note that the body contains content, despite being a 503 status code. This is actually consistent with what I saw when trying to visit the page in the browser. First I was sent to this "anti-DDoS" page you see in the response above, and then I was automatically redirected to the page requested in the URL (apparently via JavaScript). This explains why it doesn't behave as you expect outside your browser; a Python web request won't execute JavaScript to perform the redirect.
So it's definitely the service. You'll have to consult the people who made it to find out why and how they expect you to deal with it. You may want to look into whether they have a different endpoint for API calls, or the endpoint might respond differently if you set the Accept header. (application/json can be used to indicate you want JSON back.)
