Use readthedocs.yaml file as we need a recent version of sphinx

docs/environment.yml

@@ -0,0 +1,9 b''
+name: ipython_docs
+dependencies:
+- python=3.6
+- ipykernel
+- setuptools>=18.5
+- sphinx>=1.8
+- sphinx_rtd_theme
+- pip:
+    - docrepr

readthedocs.yml

@@ -0,0 +1,4 b''
+conda:
+    file: docs/environment.yml
+python:
+    version: 3