Replace download_as_string by download_as_bytes
download_as_string is deprecated since google-cloud-storage 1.31.0 see https://github.com/googleapis/python-storage/commit/73107c35f23c4a358e957c2b8188300a7fa958fe
download_as_string is deprecated since google-cloud-storage 1.31.0 see https://github.com/googleapis/python-storage/commit/73107c35f23c4a358e957c2b8188300a7fa958fe