build: bump ansys-sphinx-theme from 1.2.6 to 1.3.1

Bumps [ansys-sphinx-theme](https://github.com/ansys/ansys-sphinx-theme) from 1.2.6 to 1.3.1.
- [Release notes](https://github.com/ansys/ansys-sphinx-theme/releases)
- [Commits](ansys/ansys-sphinx-theme@v1.2.6...v1.3.1)

---
updated-dependencies:
- dependency-name: ansys-sphinx-theme
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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