Update Python version for RTD build

.readthedocs.yml

@@ -4,17 +4,21 @@
 # Required
 version: 2
 
+# Set the version of Python and other tools you might need
+build:
+  os: ubuntu-20.04
+  tools:
+    python: "3.9"
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/conf.py
 
-
 # Disabling all unneeded formats (note that HTML and JSON are always built)
 # See https://docs.readthedocs.io/en/latest/yaml-config.html#formats
 formats: []
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.7
   install:
     - requirements: requirements.txt