Bump sphinx-copybutton from 0.5.0 to 0.5.1 in /docs (#3390)

Bumps [sphinx-copybutton](https://github.com/executablebooks/sphinx-copybutton) from 0.5.0 to 0.5.1.
- [Release notes](https://github.com/executablebooks/sphinx-copybutton/releases)
- [Changelog](https://github.com/executablebooks/sphinx-copybutton/blob/master/CHANGELOG.md)
- [Commits](https://github.com/executablebooks/sphinx-copybutton/compare/v0.5.0...v0.5.1

)

---
updated-dependencies:
- dependency-name: sphinx-copybutton
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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