Update python deps to fix dependabot alert 5 (#181)

I ran `bazel run //:requirements.update -- --upgrade`.

To test this, I read the release notes for mkdocs and mkdocs material:

- https://www.mkdocs.org/about/release-notes/#version-153-2023-09-18 -
Changed from 1.5.1 to 1.5.3
- https://squidfunk.github.io/mkdocs-material/changelog/ - Changed from
9.1.21 to 9.4.6

(Our other "main" dependency, mike, was not updated.)

I also ran `au-docs-serve` and spot checked a variety of pages using an
A/B blink comparison. The only changes I found were:

- Indentation in nav bar is slightly different.
- For tabs, the highlight color of the active tab title is more muted.