chore(python): update release.sh to use keystore (#333)

Source-Link: https://github.com/googleapis/synthtool/commit/69fda12e2994f0b595a397e8bb6e3e9f380524eb
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:ae600f36b6bc972b368367b6f83a1d91ec2c82a4a116b383d67d547c56fe6de3

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>