chore(deps): update precommit hook pycqa/flake8 to v3.9.1 (#12)

Co-authored-by: Jeffrey Rennie <[email protected]>

Source-Author: WhiteSource Renovate <[email protected]>
Source-Date: Mon Apr 26 21:54:53 2021 +0200
Source-Repo: googleapis/synthtool
Source-Sha: a648ce0ca3fe122b3186d7e6861cb641437e485e
Source-Link: googleapis/synthtool@a648ce0

Co-authored-by: Dan Lee <[email protected]>