diff --git a/s3_storage_provider.py b/s3_storage_provider.py
index e0ab67e..f5b9308 100644
--- a/s3_storage_provider.py
+++ b/s3_storage_provider.py
@@ -33,8 +33,12 @@ logger = logging.getLogger("synapse.s3")
 
 
 # The list of valid AWS storage class names
-_VALID_STORAGE_CLASSES = ('STANDARD', 'REDUCED_REDUNDANCY',
-                          'STANDARD_IA', 'INTELLIGENT_TIERING')
+_VALID_STORAGE_CLASSES = (
+    "STANDARD",
+    "REDUCED_REDUNDANCY",
+    "STANDARD_IA",
+    "INTELLIGENT_TIERING",
+)
 
 # Chunk size to use when reading from s3 connection in bytes
 READ_CHUNK_SIZE = 16 * 1024
@@ -70,15 +74,13 @@ class S3StorageProviderBackend(StorageProvider):
 
         def _store_file():
             session = boto3.session.Session()
-            session.resource('s3', **self.api_kwargs).Bucket(self.bucket).upload_file(
+            session.resource("s3", **self.api_kwargs).Bucket(self.bucket).upload_file(
                 Filename=os.path.join(self.cache_directory, path),
                 Key=path,
                 ExtraArgs={"StorageClass": self.storage_class},
             )
 
-        return make_deferred_yieldable(
-            reactor.callInThread(_store_file)
-        )
+        return make_deferred_yieldable(reactor.callInThread(_store_file))
 
     def fetch(self, path, file_info):
         """See StorageProvider.fetch"""
@@ -146,12 +148,12 @@ class _S3DownloadThread(threading.Thread):
         if not hasattr(local_data, "b3_session"):
             local_data.b3_session = boto3.session.Session()
         session = local_data.b3_session
-        s3 = session.client('s3', **self.api_kwargs)
+        s3 = session.client("s3", **self.api_kwargs)
 
         try:
             resp = s3.get_object(Bucket=self.bucket, Key=self.key)
         except botocore.exceptions.ClientError as e:
-            if e.response['Error']['Code'] in ("404", "NoSuchKey",):
+            if e.response["Error"]["Code"] in ("404", "NoSuchKey",):
                 reactor.callFromThread(self.deferred.callback, None)
                 return
 
@@ -160,7 +162,7 @@ class _S3DownloadThread(threading.Thread):
 
         producer = _S3Responder()
         reactor.callFromThread(self.deferred.callback, producer)
-        _stream_to_producer(reactor, producer, resp["Body"], timeout=90.)
+        _stream_to_producer(reactor, producer, resp["Body"], timeout=90.0)
 
 
 def _stream_to_producer(reactor, producer, body, status=None, timeout=None):
@@ -219,6 +221,7 @@ def _stream_to_producer(reactor, producer, body, status=None, timeout=None):
 class _S3Responder(Responder):
     """A Responder for S3. Created by _S3DownloadThread
     """
+
     def __init__(self):
         # Triggered by responder when more data has been requested (or
         # stop_event has been triggered)
diff --git a/setup.py b/setup.py
index a7255c5..a880b1e 100644
--- a/setup.py
+++ b/setup.py
@@ -3,18 +3,18 @@ from setuptools import setup
 __version__ = "1.0"
 
 setup(
-    name='synapse-s3-storage-provider',
+    name="synapse-s3-storage-provider",
     version=__version__,
     zip_safe=False,
     url="https://github.com/matrix-org/synapse-s3-storage-provider",
-    py_modules=['s3_storage_provider'],
-    scripts=['scripts/s3_media_upload'],
+    py_modules=["s3_storage_provider"],
+    scripts=["scripts/s3_media_upload"],
     install_requires=[
-        'boto3>=1.9.23<2.0',
-        'botocore>=1.12.23<2.0',
-        'humanize>=0.5.1<0.6',
-        'psycopg2>=2.7.5<3.0',
-        'PyYAML>=3.13<4.0',
-        'tqdm>=4.26.0<5.0'
+        "boto3>=1.9.23<2.0",
+        "botocore>=1.12.23<2.0",
+        "humanize>=0.5.1<0.6",
+        "psycopg2>=2.7.5<3.0",
+        "PyYAML>=3.13<4.0",
+        "tqdm>=4.26.0<5.0",
     ],
 )
diff --git a/test_s3.py b/test_s3.py
index b7a1f61..5328ad0 100644
--- a/test_s3.py
+++ b/test_s3.py
@@ -19,7 +19,8 @@ from twisted.test.proto_helpers import MemoryReactorClock
 from twisted.trial import unittest
 
 import sys
-is_py2 = sys.version[0] == '2'
+
+is_py2 = sys.version[0] == "2"
 if is_py2:
     from Queue import Queue
 else:
@@ -29,9 +30,7 @@ from threading import Event, Thread
 
 from mock import Mock
 
-from s3_storage_provider import (
-    _stream_to_producer, _S3Responder, _ProducerStatus,
-)
+from s3_storage_provider import _stream_to_producer, _S3Responder, _ProducerStatus
 
 
 class StreamingProducerTestCase(unittest.TestCase):
@@ -52,10 +51,7 @@ class StreamingProducerTestCase(unittest.TestCase):
         self.thread = Thread(
             target=_stream_to_producer,
             args=(self.reactor, self.producer, self.body),
-            kwargs={
-                "status": self.producer_status,
-                "timeout": 1.0,
-            },
+            kwargs={"status": self.producer_status, "timeout": 1.0},
         )
         self.thread.daemon = True
         self.thread.start()
@@ -94,12 +90,12 @@ class StreamingProducerTestCase(unittest.TestCase):
         self.producer.pauseProducing()
         self.body.write(" string")
         self.wait_for_thread()
-        self.producer_status.wait_until_paused(10.)
+        self.producer_status.wait_until_paused(10.0)
         self.assertEqual("test string", self.written)
 
         # If we write again we remain paused and nothing gets written
         self.body.write(" second")
-        self.producer_status.wait_until_paused(10.)
+        self.producer_status.wait_until_paused(10.0)
         self.assertEqual("test string", self.written)
 
         # If we call resumeProducing the buffered data gets read and written.
@@ -165,6 +161,7 @@ class ThreadedMemoryReactorClock(MemoryReactorClock):
 class Channel(object):
     """Simple channel to mimic a thread safe file like object
     """
+
     def __init__(self):
         self._queue = Queue()
 
diff --git a/tox.ini b/tox.ini
index 21ade6c..ad5cc67 100644
--- a/tox.ini
+++ b/tox.ini
@@ -46,7 +46,11 @@ skip_install = True
 basepython = python3.6
 deps =
     flake8
-commands = /bin/sh -c "flake8 s3_storage_provider.py setup.py {env:PEP8SUFFIX:}"
+    # We pin so that our tests don't start failing on new releases of black.
+    black==19.10b0
+commands =
+    python -m black --check --diff .
+    /bin/sh -c "flake8 s3_storage_provider.py setup.py {env:PEP8SUFFIX:}"
 
 [testenv:check_isort]
 skip_install = True