Fix conda install on Readthedocs

[cdeil]

I tried a few things, but so far didn't manage to resolve the conda install issue on Readthedocs for hips: https://readthedocs.org/projects/hips/builds/5477266/

Looks like it's a known issue, I've left a comment there:
readthedocs/readthedocs.org#2584 (comment)

This is not urgent, we only need to find a solution for this within the next few weeks, i.e. when we start to have useful functionality. Locally, one can build the docs with python setup.py build_docs, so one can work on the docs already of course.

[cdeil]

Looks like there's a beta version supporting Python 3.6:
readthedocs/readthedocs.org#1990 (comment)
Not sure if it works with conda, I've signed up anyways and will try.

[cdeil]

I'm closing this old issue now, and will file a reminder issue to move the hips package CI and docs build from Python 3.5 -> 3.6 a bit later.

A while ago I changed the RTD build to Python 3.5 (because they didn't support Python 3.6 yet and there's missing healpy packages for python 3.6). This is the latest RTD build and it passed: https://readthedocs.org/projects/hips/builds/5556368/