Update ReadTheDocs configuration to avoid deprecation warnings with

ReadTheDocs.

.readthedocs.yaml

@@ -1,9 +1,16 @@
+# required
 version: 2
 
+build:
+  os: ubuntu-20.04
+  tools:
+    python: "3.9"
+
+# Build documentation in the doc/src directory with Sphinx
 sphinx:
    configuration: doc/src/conf.py
 
+# declare Python requirements required to build docs
 python:
-  version: 3.8
    install:
    - requirements: doc/requirements.txt

doc/requirements.txt

@@ -1,2 +1,2 @@
-sphinx==4.1.2
+sphinx>=4.2.0
-sphinx_rtd_theme==0.5.1
+sphinx-rtd-theme>=0.5.2