DOCS-#4072: Build docs with Python 3.8 (#4073)

Signed-off-by: Devin Petersohn <devin.petersohn@gmail.com>

.readthedocs.yaml

@@ -0,0 +1,20 @@
+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Set the version of Python and other tools you might need
+build:
+  os: ubuntu-20.04
+  tools:
+    python: "3.8"
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+   configuration: docs/conf.py
+
+python:
+   install:
+   - requirements: docs/requirements-doc.txt