Fix test_io_threading to run on Python 3.6.

tests/test_io_threading.py

@@ -18,7 +18,6 @@
 import random
 import threading
 from concurrent.futures import ThreadPoolExecutor
-from contextlib import nullcontext
 from functools import lru_cache
 from io import BytesIO
 
@@ -28,6 +27,11 @@
 from pedalboard import Resample
 from pedalboard.io import AudioFile
 
+try:
+    from contextlib import nullcontext
+except ImportError:
+    from contextlib import suppress as nullcontext
+
 # The number of iterations to run the test for with and
 # without a lock around each call to AudioFile's methods:
 MAX_ITERATIONS = {"lock": 10, "no lock": 100_000}