build(deps-dev): bump sphinxcontrib-bibtex in /backend

Bumps [sphinxcontrib-bibtex](https://github.com/mcmtroffaes/sphinxcontrib-bibtex) from 2.6.3 to 2.6.4.
- [Changelog](https://github.com/mcmtroffaes/sphinxcontrib-bibtex/blob/develop/CHANGELOG.rst)
- [Commits](mcmtroffaes/sphinxcontrib-bibtex@2.6.3...2.6.4)

---
updated-dependencies:
- dependency-name: sphinxcontrib-bibtex
  dependency-version: 2.6.4
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]