Fixed #21188 -- Introduced subclasses for to-be-removed-in-django-XX warnings

Thanks Anssi Kääriäinen for the idea and Simon Charette for the
review.

django/core/management/base.py

@@ -17,6 +17,7 @@ import django
 from django.core import checks
 from django.core.exceptions import ImproperlyConfigured
 from django.core.management.color import color_style, no_style
+from django.utils.deprecation import RemovedInDjango19Warning
 from django.utils.encoding import force_str
 
 
@@ -219,7 +220,7 @@ class BaseCommand(object):
             warnings.warn(
                 '"requires_model_validation" is deprecated '
                 'in favor of "requires_system_checks".',
-                PendingDeprecationWarning)
+                RemovedInDjango19Warning)
         if has_old_option and has_new_option:
             raise ImproperlyConfigured(
                 'Command %s defines both "requires_model_validation" '
@@ -467,7 +468,7 @@ class AppCommand(BaseCommand):
             warnings.warn(
                 "AppCommand.handle_app() is superseded by "
                 "AppCommand.handle_app_config().",
-                PendingDeprecationWarning, stacklevel=2)
+                RemovedInDjango19Warning, stacklevel=2)
             if app_config.models_module is None:
                 raise CommandError(
                     "AppCommand cannot handle app '%s' in legacy mode "