Pin python version to 3.5 for RTD so that docs will build

[mpacer]

I'm not entirely sure why, but when readthedocs attempts to use python 3.6 to build our docs, it fails.

I think this may relate to dependencies that do not expect to be interpreted via python 3.6. See the error log at https://readthedocs.org/projects/nbconvert/builds/4854852/.

Or the issue I filed with conda at conda/conda#4168

What really confuses me though is that this included some dependencies that list 3.6* as a potential version number… The only reason I thought to notice this is that I saw py36_ in the installation file names.

Solution: pinning the version at 3.5 until we figure out how to better solve this problem. (Note, the rtd build succeeds with this change, see https://readthedocs.org/projects/nbconvert-mp/builds/4855978/)

[mpacer]

Hmm this fixed this for my personal repo but not the jupyter one.

[mpacer]

Oh actually it did work! It just was the last to be merged so the other two merges still failed & sent emails saying so.