update readthedocs config

.readthedocs.yml

@@ -5,6 +5,12 @@
 # Required
 version: 2
 
+# Set the OS, Python version and other tools you might need
+build:
+  os: ubuntu-lts-latest
+  tools:
+    python: "3.9"
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/conf.py
@@ -18,7 +24,6 @@ formats: all
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.7
   install:
     - requirements: requirements.txt
     - method: pip