DOC: Update RTD env

[ci skip]

.readthedocs.yml

@@ -2,15 +2,21 @@
 # See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
 version: 2
 
+build:
+  os: ubuntu-20.04
+  apt_packages:
+    - graphviz
+  tools:
+    python: "3.9"
+
 sphinx:
   builder: html
   configuration: docs/conf.py
   fail_on_warning: true
 
 # Set the version of Python and requirements required to build your docs
 python:
-  version: 3.7
-  system_packages: true
+  system_packages: false
   install:
     - method: pip
       path: .