⬆️ Bump sphinx-proof from 0.3.0 to 0.4.0 (#166)

Bumps [sphinx-proof](https://github.com/executablebooks/sphinx-proof) from 0.3.0 to 0.4.0.
- [Release notes](https://github.com/executablebooks/sphinx-proof/releases)
- [Changelog](https://github.com/executablebooks/sphinx-proof/blob/main/CHANGELOG.md)
- [Commits](executablebooks/sphinx-proof@v0.3.0...v0.4.0)

---
updated-dependencies:
- dependency-name: sphinx-proof
  dependency-version: 0.4.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

environment.yml

@@ -9,7 +9,7 @@ dependencies:
     - jupyter-book==1.0.4post1
     - quantecon-book-theme==0.15.1
     - sphinx-tojupyter==0.6.0
-    - sphinx-proof==0.3.0
+    - sphinx-proof==0.4.0
     - sphinxext-rediraffe==0.3.0
     - sphinx-exercise==1.2.1
     - sphinxcontrib-youtube==1.4.1