Bump sphinx-gallery from 0.20.0 to 0.21.0 (#265)

Bumps [sphinx-gallery](https://github.com/sphinx-gallery/sphinx-gallery) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/sphinx-gallery/sphinx-gallery/releases)
- [Changelog](https://github.com/sphinx-gallery/sphinx-gallery/blob/master/CHANGES.rst)
- [Commits](sphinx-gallery/sphinx-gallery@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: sphinx-gallery
  dependency-version: 0.21.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]