Update docs/conf.py, define html_context variable

Co-authored-by: jakirkham <jakirkham@gmail.com>

docs/conf.py

@@ -123,6 +123,7 @@
 # Set canonical URL from the Read the Docs Domain
 html_baseurl = os.environ.get("READTHEDOCS_CANONICAL_URL", "")
 
+html_context = {}
 # Tell Jinja2 templates the build is running on Read the Docs
 if os.environ.get("READTHEDOCS", "") == "True":
     html_context["READTHEDOCS"] = True