feat: add concept of (non-)default importers for handler

i.e. when telling handler to "import all" which should be included
vs. excluded by default?

tests/importing/test_handlers.py

@@ -7,6 +7,7 @@ from uuid import UUID
 from wuttjamaican.testing import DataTestCase
 
 from wuttasync.importing import handlers as mod, Importer, ToSqlalchemy
+from wuttasync.importing.wutta import FromWuttaToWuttaImport
 
 
 class FromFooToBar(mod.ImportHandler):
@@ -253,6 +254,25 @@ class TestImportHandler(DataTestCase):
             KeyError, handler.get_importer, "BunchOfNonsense", model_class=model.Setting
         )
 
+    def test_is_default(self):
+        handler = self.make_handler()
+        # nb. anything is considered default, by default
+        self.assertTrue(handler.is_default("there_is_no_way_this_is_valid"))
+
+    def test_get_default_importer_keys(self):
+
+        # use handler which already has some non/default keys
+        handler = FromWuttaToWuttaImport(self.config)
+
+        # it supports many importers
+        self.assertIn("Person", handler.importers)
+        self.assertIn("User", handler.importers)
+        self.assertIn("Setting", handler.importers)
+
+        # but only Person is default
+        keys = handler.get_default_importer_keys()
+        self.assertEqual(keys, ["Person"])
+
     def test_get_warnings_email_key(self):
         handler = FromFooToBar(self.config)