Install terminado for docs build

RTD is currently failing with an ImportError

Also move to a recent Python, because I don't know how long 3.5 will be available on conda.

Author: takluyver

docs/environment.yml

@@ -13,4 +13,5 @@ dependencies:
   - sphinxcontrib_github_alt
   - sphinxcontrib-openapi
   - sphinxemoji
+  - terminado
   - git+https://github.com/pandas-dev/pydata-sphinx-theme.git@master