Filter apps when looping in system check

Improves readability by declaring that this filter is not part of the
system check, but limits the system check to a subset of apps.

aleksis/core/checks.py

@@ -17,8 +17,8 @@ def check_app_configs_base_class(
     if app_configs is None:
         app_configs = django.apps.apps.get_app_configs()
 
-    for app_config in app_configs:
-        if app_config.name.startswith("aleksis.apps.") and not isinstance(app_config, AppConfig):
+    for app_config in filter(lambda c: c.name.startswith("aleksis.apps."), app_configs):
+        if not isinstance(app_config, AppConfig):
             results.append(
                 Warning(
                     "App config %s does not derive from aleksis.core.util.apps.AppConfig.",