Sync from Piper @305505267

PROTOBUF_SYNC_PIPER
diff --git a/python/setup.py b/python/setup.py
index 9aabbf7..7e462b1 100755
--- a/python/setup.py
+++ b/python/setup.py
@@ -110,6 +110,7 @@
   generate_proto("google/protobuf/internal/no_package.proto", False)
   generate_proto("google/protobuf/internal/packed_field_test.proto", False)
   generate_proto("google/protobuf/internal/test_bad_identifiers.proto", False)
+  generate_proto("google/protobuf/internal/test_proto3_optional.proto", False)
   generate_proto("google/protobuf/pyext/python.proto", False)