Missing dependencies for astropy docs build

I just did the experiment of following the instructions at:
https://astropy.readthedocs.io/en/latest/install.html#building-documentation

These instructions are really pretty good (simple!) and seem most of the way to allowing newcomers to make and test documentation updates. But...

I made a clean conda python=3.7 environment and a fresh astropy clone and did:
```
conda install -c astropy sphinx-astropy
python setup.py build_docs
```
It looks like the following dependencies for building astropy docs are missed:
- Cython (this gets pip installed, but still...)
- pytest (build process exception `ModuleNotFoundError: No module named 'pytest'`)
- pyyaml (module not found exception)
- scipy (build seemed to get stuck at some 95% reading source; when I ctrl-c'd at 25 CPU minutes it complained about scipy import errors)
- skimage (After fixing the previous issues, I left the doc build going and stopped at 1.5 hrs CPU, and it showed an exception ModuleNotFoundError: No module named 'skimage')