RTD fails with WARNING: undefined label: 'python:bltin-boolean-values'

- [ ] Revert https://github.com/astropy/astropy/pull/15433
- [ ] Rebuild doc for v5.3.4

<hr/>

Example log: https://readthedocs.org/projects/astropy/builds/22123170/

```
WARNING: undefined label: 'python:bltin-boolean-values'
```

729 warnings 😱 

This happens out of nowhere, so might have to ask RTD people first.