update quantecon-book-theme to 0.20.1

[mmcky]

Update quantecon-book-theme

This PR updates the theme to test the citation emphasis fix from QuantEcon/quantecon-book-theme#378.

What's being tested

The theme's emphasis styling (em elements display in teal color instead of italics) was incorrectly applied to "et al." text in sphinxcontrib-bibtex textual citations (:cite:t: with author_year style).

The fix adds a CSS :has() selector to match the HTML structure where <em>et al.</em> is a sibling of the <a class="reference"> link, not a child.

Preview pages to check

• doubts_or_variability — has multiple Barillas et al. [2009] citations

Next steps

Once the preview build confirms the fix works:

1. Merge quantecon-book-theme#378
2. Release quantecon-book-theme v0.20.2
3. Update this PR to pin quantecon-book-theme==0.20.2
4. Merge this PR

[Copilot]

Pull request overview

Updates the project build environment to use quantecon-book-theme v0.20.1 to pick up the latest theme fixes (notably citation link emphasis styling) and dependency compatibility improvements.

Changes:

• Bump quantecon-book-theme from 0.19.0 to 0.20.1 in the conda/pip environment definition.

[Copilot]

This bumps quantecon-book-theme in environment.yml, but the scheduled .github/workflows/collab.yml workflow installs its own pinned quantecon-book-theme==0.8.2 (bypassing this environment). That means the Colab execution check won’t reflect the theme update and may render/test citations differently. Consider updating that workflow to use environment.yml (or at least bump the pinned theme version) so all builds validate the same dependency set.

[github-actions]

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Commit: 27e542b

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Build Info

• Workflow: Build Project [using jupyter-book]

[mmcky]

Closing as tests are completed.