Second try to fix readthedocs on master.

docs/environment.yml

@@ -1,14 +1,15 b''
 name: ipython_docs
 dependencies:
 - python=3.6
 - setuptools>=18.5
 - sphinx>=1.8
 - sphinx_rtd_theme
 - numpy
 - nose
 - testpath
 - matplotlib
+- stack_data
 - pip:
     - docrepr
     - prompt_toolkit
     - ipykernel