chore(python): add requirements for docs build

[gcf-owl-bot[bot]]

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

[vchudnov-g]

Some of the version numbers have gone down, which I don't understand. Not merging until we investigate.

[parthea]

Closing as obsolete