fix rtd build

.readthedocs.yaml

@@ -4,6 +4,10 @@
 
 # Required
 version: 2
+build:
+  os: "ubuntu-22.04"
+  tools:
+    python: "3.8"
 
 # Build documentation in the docs/ directory with Sphinx
 # sphinx:
@@ -18,6 +22,6 @@ formats: all
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.7
+  version: "3.8"
   install:
     - requirements: requirements/docs.txt