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

@@ -1,13 +1,14 @@
 name: notebook_docs
 dependencies:
-- python=3.5
+- python=3.8
 - sphinx_rtd_theme
 - jinja2
 - tornado
 - nbformat
 - jupyter_client
 - ipykernel
 - sphinx
+- terminado
 - pip:
   - nbsphinx
   - Send2Trash