test: add missing return type hint in test method

Following parallel merges of theupdateframework#1700 (added new test method),
and theupdateframework#1710 (started running mypy on tests), ci/cd fails in the
develop branch. This is fixed in this patch.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

tests/test_examples.py

@@ -87,7 +87,7 @@ def test_basic_repo(self) -> None:
             ],
         )
 
-    def test_hashed_bin_delegation(self):
+    def test_hashed_bin_delegation(self) -> None:
         """Run 'hashed_bin_delegation.py' and assert creation of metadata files."""
         self._run_script_and_assert_files(
             "hashed_bin_delegation.py",