backport new environment file and install instructions

[drammock]

@larsoner not sure if I did the cherry-pick correctly; the diff ended up including some changes that weren't from #7971 ... but I think that's actually what we want in this case?

[larsoner]

Technically we don't want to bump Travis on maint/0.20 to 3.6 because 0.20 still supports 3.5. But it probably does not matter much since we're releasing 0.21 in about two months, so if it's enough to make the CIs happy then I guess it's fine.

[drammock]

hmm, circle failures are all related to _dict stuff. I guess I need to cherry-pick the ignore-nitpicky lines in doc/conf.py?

[larsoner]

Yep, that should do it

[drammock]

@larsoner Circle is happy, and I've reverted all the changes except for the changes to the docs, and the nitpicky ignores in conf.py. Note that I had to add a new nitpick ignore to make Circle happy. Is this OK to merge then, to get the stable docs in sync with the current state of environment.yml?

[larsoner]

Thanks @drammock