When Django validates the host from the HTTP request it attempts to match it against the following regex which you can find in the source here:
host_validation_re = re.compile(r"^([a-z0-9.-]+|\[[a-f0-9]*:[a-f0-9:]+\])(:\d+)?$")
The underscore in your username is causing Django to reject the host name on the request. You can create a new PythonAnywhere account with a new username or use a different web framework.