Use new pytest api to keep building with pytest 5 (#353)

diff --git a/tests/test__default.py b/tests/test__default.py
index 3fb0fa1..d143479 100644
--- a/tests/test__default.py
+++ b/tests/test__default.py
@@ -92,7 +92,7 @@
 
 
 def test__load_credentials_from_file_authorized_user_cloud_sdk():
-    with pytest.warns(UserWarning, matches='Cloud SDK'):
+    with pytest.warns(UserWarning, match='Cloud SDK'):
         credentials, project_id = _default._load_credentials_from_file(
             AUTHORIZED_USER_CLOUD_SDK_FILE)
     assert isinstance(credentials, google.oauth2.credentials.Credentials)