⬆️ Bump quantecon-book-theme from 0.18.0 to 0.19.0 (#64)

Bumps [quantecon-book-theme](https://github.com/QuantEcon/quantecon-book-theme) from 0.18.0 to 0.19.0.
- [Release notes](https://github.com/QuantEcon/quantecon-book-theme/releases)
- [Changelog](https://github.com/QuantEcon/quantecon-book-theme/blob/main/CHANGELOG.md)
- [Commits](QuantEcon/quantecon-book-theme@v0.18.0...v0.19.0)

---
updated-dependencies:
- dependency-name: quantecon-book-theme
  dependency-version: 0.19.0
  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

@@ -7,7 +7,7 @@ dependencies:
   - pip
   - pip:
     - jupyter-book>=1.0.4post1,<2.0
-    - quantecon-book-theme==0.18.0
+    - quantecon-book-theme==0.19.0
     - sphinx-tojupyter==0.6.0
     - sphinxext-rediraffe==0.3.0
     - sphinx-exercise==1.2.1