Add config for Read The Docs

Read the Docs can build without a configuration file, but if you want to
do anything other than the defaults, you need to provide one.

In our case, we want to build the docs using Python 3.9.  So, we bring
in a configuration file.

This might fix #1, but that can't be confirmed until docs build.

.readthedocs.yaml

@@ -0,0 +1,27 @@
+# Configuration file for Read the Docs.
+
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for the
+# configuration file reference.
+version: 2
+
+# I think it's unlikely that people will want the non-HTML docs, but
+# if that's not true, please let us know and we'll change this!
+# http://github.com/stanford-rc/mais-apis-python/issues
+formats: []
+
+# We're building using Sphinx
+sphinx:
+  configuration: docs/conf.py
+
+# Use the latest version of Python that we explicitly support.
+python:
+  version: 3.9
+  install:
+    - method: pip
+      path: .
+      extra_requirements:
+        - docs
+
+# The testing image is needed to use Python 3.9.
+build:
+  image: testing