Bump sphinx-issues from 5.0.1 to 6.0.0 (#247)

Bumps [sphinx-issues](https://github.com/sloria/sphinx-issues) from 5.0.1 to 6.0.0.
- [Commits](sloria/sphinx-issues@5.0.1...6.0.0)

---
updated-dependencies:
- dependency-name: sphinx-issues
  dependency-version: 6.0.0
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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Author: dependabot[bot]