Read the Docs gets tempest-helper from PyPI

In `.readthedocs_conda_env.yml` tempest-helper is installed through pip from PyPI rather than conda because RtD's conda install process was giving the error:

```
ResolvePackageNotFound:
- tempest-helper
```

tempest-helper is available from conda-forge and the same conda environment was not giving this error when running locally. Perhaps RtD was caching conda-forge as tempest-helper had been added relatively recently.

At some point the RtD conda environment should be updated to see if tempest-helper can be installed from conda rather than PyPI.

Start with `.readthedocs_conda_env.yml` and inspect how `tempest-helper` is currently installed from PyPI. Check whether the package is now available from conda-forge for the Read the Docs environment, then verify the documentation build completes with the conda dependency; done means the PyPI installation is no longer needed.

Written by the indexing model from the issue text.