Update conf.py, sphinx context injection deprecated in ReadTheDocs (#383

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* Update conf.py, sphinx context injection deprecated in ReadTheDocs

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

* Update docs/conf.py, define html_context variable

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

* Handle already defined `html_context`

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

docs/conf.py

@@ -120,6 +120,15 @@
 
 # -- 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":
+    if "html_context" not in globals():
+        html_context = {}
+    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'