chore(python): exclude .github/workflows/unittest.yml in renovate config (#774)

Source-Link: https://github.com/googleapis/synthtool/commit/106d292bd234e5d9977231dcfbc4831e34eba13a
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:8ff1efe878e18bd82a0fb7b70bb86f77e7ab6901fed394440b6135db0ba8d84a

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