Update conf.py, sphinx context injection deprecated in ReadTheDocs

ReadTheDocs is deprecating sphinx context injection. See details at dask#382

docs/conf.py

@@ -120,6 +120,13 @@
 
 # -- Options for HTML output -------------------------------------------
 
+# Set canonical URL from the Read the Docs Domain
+html_baseurl = os.environ.get("READTHEDOCS_CANONICAL_URL", "")
+
+# Tell Jinja2 templates the build is running on Read the Docs
+if os.environ.get("READTHEDOCS", "") == "True":
+    html_context["READTHEDOCS"] = True
+
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
 html_theme = 'dask_sphinx_theme'