chore: blacken (#375)

diff --git a/google/auth/iam.py b/google/auth/iam.py
index e091e47..a438726 100644
--- a/google/auth/iam.py
+++ b/google/auth/iam.py
@@ -28,9 +28,8 @@
 from google.auth import crypt
 from google.auth import exceptions
 
-_IAM_API_ROOT_URI = 'https://iam.googleapis.com/v1'
-_SIGN_BLOB_URI = (
-    _IAM_API_ROOT_URI + '/projects/-/serviceAccounts/{}:signBlob?alt=json')
+_IAM_API_ROOT_URI = "https://iam.googleapis.com/v1"
+_SIGN_BLOB_URI = _IAM_API_ROOT_URI + "/projects/-/serviceAccounts/{}:signBlob?alt=json"
 
 
 class Signer(crypt.Signer):
@@ -68,23 +67,20 @@
         """Makes a request to the API signBlob API."""
         message = _helpers.to_bytes(message)
 
-        method = 'POST'
+        method = "POST"
         url = _SIGN_BLOB_URI.format(self._service_account_email)
         headers = {}
-        body = json.dumps({
-            'bytesToSign': base64.b64encode(message).decode('utf-8'),
-        })
+        body = json.dumps({"bytesToSign": base64.b64encode(message).decode("utf-8")})
 
         self._credentials.before_request(self._request, method, url, headers)
-        response = self._request(
-            url=url, method=method, body=body, headers=headers)
+        response = self._request(url=url, method=method, body=body, headers=headers)
 
         if response.status != http_client.OK:
             raise exceptions.TransportError(
-                'Error calling the IAM signBytes API: {}'.format(
-                    response.data))
+                "Error calling the IAM signBytes API: {}".format(response.data)
+            )
 
-        return json.loads(response.data.decode('utf-8'))
+        return json.loads(response.data.decode("utf-8"))
 
     @property
     def key_id(self):
@@ -99,4 +95,4 @@
     @_helpers.copy_docstring(crypt.Signer)
     def sign(self, message):
         response = self._make_signing_request(message)
-        return base64.b64decode(response['signature'])
+        return base64.b64decode(response["signature"])