chore(python): add requirements for docs build (#1125)

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

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -15,3 +15,4 @@ docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-python:latest
   digest: sha256:a8a80fc6456e433df53fc2a0d72ca0345db0ddefb409f1b75b118dfd1babd952
 # created: 2024-03-15T16:25:47.905264637Z
+