Fix import of ImproperlyConfigured in app_settings

Global import does not work in app_setings files.

src/authentic2_auth_msp/app_settings.py

@@ -1,7 +1,3 @@
-import sys
-
-from django.core.exceptions import ImproperlyConfigured
-
 
 class AppSettings(object):
     '''Thanks django-allauth'''
@@ -12,6 +8,8 @@ class AppSettings(object):
 
     def _setting(self, name, dflt=__SENTINEL):
         from django.conf import settings
+        from django.core.exceptions import ImproperlyConfigured
+
         v  = getattr(settings, self.prefix + name, dflt)
         if v is self.__SENTINEL:
             raise ImproperlyConfigured('Missing setting %r' % (self.prefix + name))
@@ -58,6 +56,8 @@ class AppSettings(object):
         return self._setting('MORE_URL', 'https://mon.service-public.fr/')
 
 
+import sys
+
 app_settings = AppSettings('A2_MSP_')
 app_settings.__name__ = __name__
 sys.modules[__name__] = app_settings