Update dependency google-cloud-spanner to v1.15.1 - #3377

Update dependency google-cloud-spanner to v1.15.1#3377

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

GoogleCloudPlatform:masterGoogleCloudPlatform/python-docs-samples:masterfrom

renovate-bot:renovate/google-cloud-spanner-1.xCopy head branch name to clipboard