chore: blacken (#375)

diff --git a/tests/test_app_engine.py b/tests/test_app_engine.py
index 70fa5ca..9559a2c 100644
--- a/tests/test_app_engine.py
+++ b/tests/test_app_engine.py
@@ -25,6 +25,7 @@
     See https://cloud.google.com/appengine/docs/standard/python/refdocs
     /google.appengine.api.app_identity.app_identity
     """
+
     def get_application_id(self):
         raise NotImplementedError()
 
@@ -41,10 +42,8 @@
 @pytest.fixture
 def app_identity(monkeypatch):
     """Mocks the app_identity module for google.auth.app_engine."""
-    app_identity_module = mock.create_autospec(
-        _AppIdentityModule, instance=True)
-    monkeypatch.setattr(
-        app_engine, 'app_identity', app_identity_module)
+    app_identity_module = mock.create_autospec(_AppIdentityModule, instance=True)
+    monkeypatch.setattr(app_engine, "app_identity", app_identity_module)
     yield app_identity_module
 
 
@@ -57,13 +56,15 @@
     with pytest.raises(EnvironmentError) as excinfo:
         assert app_engine.get_project_id()
 
-    assert excinfo.match(r'App Engine APIs are not available')
+    assert excinfo.match(r"App Engine APIs are not available")
 
 
 class TestSigner(object):
     def test_key_id(self, app_identity):
         app_identity.sign_blob.return_value = (
-            mock.sentinel.key_id, mock.sentinel.signature)
+            mock.sentinel.key_id,
+            mock.sentinel.signature,
+        )
 
         signer = app_engine.Signer()
 
@@ -71,10 +72,12 @@
 
     def test_sign(self, app_identity):
         app_identity.sign_blob.return_value = (
-            mock.sentinel.key_id, mock.sentinel.signature)
+            mock.sentinel.key_id,
+            mock.sentinel.signature,
+        )
 
         signer = app_engine.Signer()
-        to_sign = b'123'
+        to_sign = b"123"
 
         signature = signer.sign(to_sign)
 
@@ -87,7 +90,7 @@
         with pytest.raises(EnvironmentError) as excinfo:
             app_engine.Credentials()
 
-        assert excinfo.match(r'App Engine APIs are not available')
+        assert excinfo.match(r"App Engine APIs are not available")
 
     def test_default_state(self, app_identity):
         credentials = app_engine.Credentials()
@@ -106,52 +109,52 @@
         assert not credentials.scopes
         assert credentials.requires_scopes
 
-        scoped_credentials = credentials.with_scopes(['email'])
+        scoped_credentials = credentials.with_scopes(["email"])
 
-        assert scoped_credentials.has_scopes(['email'])
+        assert scoped_credentials.has_scopes(["email"])
         assert not scoped_credentials.requires_scopes
 
     def test_service_account_email_implicit(self, app_identity):
         app_identity.get_service_account_name.return_value = (
-            mock.sentinel.service_account_email)
+            mock.sentinel.service_account_email
+        )
         credentials = app_engine.Credentials()
 
-        assert (credentials.service_account_email ==
-                mock.sentinel.service_account_email)
+        assert credentials.service_account_email == mock.sentinel.service_account_email
         assert app_identity.get_service_account_name.called
 
     def test_service_account_email_explicit(self, app_identity):
         credentials = app_engine.Credentials(
-            service_account_id=mock.sentinel.service_account_email)
+            service_account_id=mock.sentinel.service_account_email
+        )
 
-        assert (credentials.service_account_email ==
-                mock.sentinel.service_account_email)
+        assert credentials.service_account_email == mock.sentinel.service_account_email
         assert not app_identity.get_service_account_name.called
 
-    @mock.patch(
-        'google.auth._helpers.utcnow',
-        return_value=datetime.datetime.min)
+    @mock.patch("google.auth._helpers.utcnow", return_value=datetime.datetime.min)
     def test_refresh(self, utcnow, app_identity):
-        token = 'token'
+        token = "token"
         ttl = 643942923
         app_identity.get_access_token.return_value = token, ttl
-        credentials = app_engine.Credentials(scopes=['email'])
+        credentials = app_engine.Credentials(scopes=["email"])
 
         credentials.refresh(None)
 
         app_identity.get_access_token.assert_called_with(
-            credentials.scopes, credentials._service_account_id)
+            credentials.scopes, credentials._service_account_id
+        )
         assert credentials.token == token
-        assert credentials.expiry == datetime.datetime(
-            1990, 5, 29, 1, 2, 3)
+        assert credentials.expiry == datetime.datetime(1990, 5, 29, 1, 2, 3)
         assert credentials.valid
         assert not credentials.expired
 
     def test_sign_bytes(self, app_identity):
         app_identity.sign_blob.return_value = (
-            mock.sentinel.key_id, mock.sentinel.signature)
+            mock.sentinel.key_id,
+            mock.sentinel.signature,
+        )
         credentials = app_engine.Credentials()
-        to_sign = b'123'
+        to_sign = b"123"
 
         signature = credentials.sign_bytes(to_sign)