Fix get_metadata() method call

Fix the following mypy error:

```
lib/tool_shed/test/functional/test_0000_basic_repository_features.py:303: error: Item "None" of "ToolShedPopulator | None" has no attribute "get_metadata"  [union-attr]
            metadata = self._populator.get_metadata(repository, downloadable_only=False)
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

lib/tool_shed/test/functional/test_0000_basic_repository_features.py

@@ -300,7 +300,7 @@ def test_0110_delete_filtering_repository(self):
         repository = self._get_repository_by_name_and_owner(repository_name, common.test_user_1_name)
         self.login(email=common.admin_email, username=common.admin_username)
         self.delete_repository(repository)
-        metadata = self._populator.get_metadata(repository, downloadable_only=False)
+        metadata = self.populator.get_metadata(repository, downloadable_only=False)
         for _, value in metadata.__root__.items():
             assert not value.downloadable
         # Explicitly reload all metadata revisions from the database, to ensure that we have the current status of the downloadable flag.