Update jupyter-book requirement from <2 to <3

Updates the requirements on [jupyter-book](https://github.com/jupyter-book/jupyter-book) to permit the latest version.
- [Release notes](https://github.com/jupyter-book/jupyter-book/releases)
- [Commits](jupyter-book/jupyter-book@v0.1...v2.1.4)

---
updated-dependencies:
- dependency-name: jupyter-book
  dependency-version: 2.1.4
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -1,5 +1,5 @@
 # docs
-jupyter-book<2
+jupyter-book<3
 git+https://github.com/executablebooks/sphinx-book-theme.git
 sphinx-copybutton
 sphinx-prompt