REV: restore setuptools to latest version on readthedocs

[rrjbca]

This reverts PR #493. To be merged after a new speclite release fixing the incompatibility with setuptools>=58.0.2

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [ ] Write documentation strings
• [ ] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request