Sphinx: use html_baseurl for setting the canonical URL

Fixes readthedocs/readthedocs-sphinx-ext#83

readthedocs/doc_builder/templates/doc_builder/conf.py.tmpl

@@ -120,6 +120,14 @@ context = {
     'commit': {% if project.repo_type == 'git' %}'{{ commit|slice:"8" }}'{% else %}'{{ commit }}'{% endif %},
 }
 
+# For sphinx >=1.8 we can use html_baseurl to set the canonical URL.
+# https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-html_baseurl
+if version_info >= (1, 8):
+    if not globals().get('html_baseurl'):
+        html_baseurl = context['canonical_url']
+    context['canonical_url'] = None
+
+
 {# Provide block for extending context data from child template #}
 {% block extra_context %}{% endblock %}