Fix broken stylesheet in the docs (#477)

The bullet points aren't rendering correctly for some reason.

Updating the theme version seemed to fix pygae/clifford, so we do the same here.

Something else broke the nbsphinx prolog, so we copy the fix from pygae/clifford for that too.

doc/conf.py

@@ -75,9 +75,14 @@
 nbsphinx_timeout = 60
 
 # This is processed by Jinja2 and inserted before each notebook
+# Some change in dependencies made us need to replace `var` with
+# `env.config.html_context['var']`.
 nbsphinx_prolog = r"""
 {% set docname = 'doc/' + env.doc2path(env.docname, base=None) %}
-{% set git_ref = 'master' if '.' not in env.config.current_version else 'v' + env.config.release %}
+{% set git_ref = 'master' if not env.config.html_context['READTHEDOCS'] else
+                 env.config.html_context['github_version']
+                 if '.' not in env.config.html_context['current_version'] else
+                 'v' + env.config.release %}
 .. raw:: html
 
     <div class="admonition note">

doc/readthedocs-pip-requirements.txt

@@ -10,7 +10,7 @@ scipy
 ipykernel
 nbsphinx
 numba
-sphinx_rtd_theme
+sphinx_rtd_theme>=0.5.0
 sphinx-markdown-tables
 notedown
 semantic-version==2.6.0