chore(python): add requirements for docs build (#356)

Source-Link: googleapis/synthtool@85c23b6
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>
Co-authored-by: Victor Chudnovsky <vchudnov@google.com>

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -13,5 +13,6 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-python:latest
-  digest: sha256:007e7e46ef05e5a32e652bd0062be02f6ff050347d91e0f357b28caab0a042c4
-# created: 2024-03-15T14:27:15.879623611Z
+  digest: sha256:3741fd1f5f5150378563c76afa06bcc12777b5fe54c5ee01115218f83872134f
+# created: 2024-03-15T16:26:15.743347415Z
+