Fix lint error (#289)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

nbclient/tests/test_util.py

@@ -38,7 +38,7 @@ def test_nested_asyncio_with_tornado():
     async def some_async_function():
         future: asyncio.Future = asyncio.ensure_future(asyncio.sleep(0.1))
         # the asyncio module, check if tornado likes it:
-        ioloop.add_future(future, lambda f: f.result())  # type:ignore
+        ioloop.add_future(future, lambda f: f.result())
         await future
         return 42