Fixed Python 3.2/3.3 compatibility

src/pretix/urls.py

@@ -1,4 +1,4 @@
-import importlib.util
+import importlib
 
 from django.apps import apps
 from django.conf import settings
@@ -6,6 +6,7 @@ from django.conf.urls import include, url
 
 import pretix.control.urls
 import pretix.presale.urls
+from pretix.helpers.importlib import module_exists
 
 urlpatterns = [
     url(r'^control/', include(pretix.control.urls, namespace='control')),
@@ -22,7 +23,7 @@ if settings.DEBUG:
 pluginpatterns = []
 for app in apps.get_app_configs():
     if hasattr(app, 'PretixPluginMeta'):
-        if importlib.util.find_spec(app.name + '.urls'):
+        if module_exists(app.name + '.urls'):
             urlmod = importlib.import_module(app.name + '.urls')
             pluginpatterns.append(
                 url(r'', include(urlmod, namespace=app.label))