fix: enable self signed jwt for grpc#427

gcf-merge-on-green[bot] merged 2 commits into

mastergoogleapis/python-spanner:masterfrom

owl-bot-0ea39061-c6c2-458d-bb0d-410e5f1eb43agoogleapis/python-spanner:owl-bot-0ea39061-c6c2-458d-bb0d-410e5f1eb43aCopy head branch name to clipboard