Bump piccolo-theme from 0.20.0 to 0.21.0 in /requirements

Bumps [piccolo-theme](https://github.com/piccolo-orm/piccolo_theme) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/piccolo-orm/piccolo_theme/releases)
- [Commits](piccolo-orm/piccolo_theme@0.20.0...0.21.0)

---
updated-dependencies:
- dependency-name: piccolo-theme
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements/docs.txt

@@ -1,4 +1,4 @@
 jupyter-book==0.15.1
-piccolo_theme==0.20.0
+piccolo_theme==0.21.0
 sphinx-issues==4.0.0
 sphinxcontrib-programoutput==0.17