Silence expected stderr (#621)

* Silence expected stderr output during test

* Change based on PR comment

tests/test_black.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python3
 import asyncio
 from concurrent.futures import ThreadPoolExecutor
-from contextlib import contextmanager
+from contextlib import contextmanager, redirect_stderr
 from functools import partial, wraps
 from io import BytesIO, TextIOWrapper
 import os
@@ -1454,6 +1454,7 @@ async def test_blackd_py36(self) -> None:
     @unittest.skipUnless(has_blackd_deps, "blackd's dependencies are not installed")
     @async_test
     async def test_blackd_fast(self) -> None:
+        with open(os.devnull, "w") as dn, redirect_stderr(dn):
             app = blackd.make_app()
             async with TestClient(TestServer(app)) as client:
                 response = await client.post("/", data=b"ur'hello'")