These days, domain names aren't restricted to being ASCII-only. For example, let's imagine that we at PythonAnywhere wanted to open a branch in China, and call it 蟒蛇云端. In the past, because domain names had to be in ASCII, we would have been stuck with something like mangsheyunduan.cn as a domain name, which would be hard for people to understand and remember.
Nowadays, you can use Unicode for domain names -- so we could get 蟒蛇云端.cn and use it. But the underlying DNS system still uses ASCII; the trick used under the hood is the International Domain Name, or IDN, system. IDNs work by mapping non-ASCII characters into strings of standard seven-bit ASCII ones. If you register 蟒蛇云端.cn, what you're actually registering is the IDN equivalent, which is xn--9kqp59h3ohr4a.cn. Verisign have a converter page that allows you to find out the IDN equivalent of an non-ASCII domain, and vice versa.
So let's say you've bought 蟒蛇云端.cn, and want to set up a website at
http://www.蟒蛇云端.cn/ on PythonAnywhere. All you need to do is go to the
"Web" tab, create a new website using the IDN version of the domain, "www.xn--9kqp59h3ohr4a.cn",
and then proceed as normal. Once it's created, you'll get the value you
need to provide for a CNAME with your registrar, and you're all set! When
people type www.蟒蛇云端.cn into their browser, they'll see your site.