Stop using URLValidator to validate return URLs.

It's excessively picky, and in particular rejects single-component host
names.  These are permitted by the URI spec, and are generated by
django.test.RequestFactory.  In any case, the next thing we do with the
returned URL is to compare it with one we've generated internally,
which should catch any genuinely invalid URLs.

ucamwebauth/__init__.py

@@ -7,7 +7,6 @@ except ImportError:
 
 from OpenSSL.crypto import FILETYPE_PEM, load_certificate, verify
 from django.conf import settings
-from django.core.validators import URLValidator
 
 from ucamwebauth.utils import decode_sig, setting, parse_time, get_return_url
 from ucamwebauth.exceptions import (MalformedResponseError, InvalidResponseError, PublicKeyNotFoundError,
@@ -109,12 +108,7 @@ class RavenResponse(object):
             raise MalformedResponseError("Empty ID")
 
         # url: The value of url supplied in the authentication request and used to form the authentication response.
-        try:
-            self.url = tokens[5]
-            urlvalidator = URLValidator() # From django 1.7 URLValidator accepts schemes=['https', 'http']
-            urlvalidator(self.url)
-        except Exception:
-            raise MalformedResponseError("The url parameter is not a valid url, got %s" % tokens[5])
+        self.url = tokens[5]
 
         # Check that 'url' represents the resource currently being
         # accessed.  The request has already been checked against