chore(python): avoid `.nox` directories when building docs (#249)

Source-Link: https://github.com/googleapis/synthtool/commit/7e1f6da50524b5d98eb67adbf6dd0805df54233d
Post-Processor: gcr.io/repo-automation-bots/owlbot-python:latest@sha256:a1a891041baa4ffbe1a809ac1b8b9b4a71887293c9101c88e8e255943c5aec2d