plugin: unconfigure should restore not unblock

`restore` pops the `block` in `_setup_django`, while `unblock` pushes an
unblock. We want to leave things clean, so should `restore`.

Author: bluetech

pytest_django/plugin.py

@@ -477,10 +477,10 @@ def get_order_number(test: pytest.Item) -> int:
 
 
 def pytest_unconfigure(config: pytest.Config) -> None:
-    if blocking_manager_key not in config.stash:
-        return
-    blocking_manager = config.stash[blocking_manager_key]
-    blocking_manager.unblock()
+    # Undo the block() in _setup_django().
+    if blocking_manager_key in config.stash:
+        blocking_manager = config.stash[blocking_manager_key]
+        blocking_manager.restore()
 
 
 @pytest.fixture(autouse=True, scope="session")