chore(python): add requirements for docs build (#632)

Source-Link: https://github.com/googleapis/synthtool/commit/85c23b6bc4352c1b0674848eaeb4e48645aeda6b
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:3741fd1f5f5150378563c76afa06bcc12777b5fe54c5ee01115218f83872134f

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