Fix ReadTheDocs

.readthedocs.yml

@@ -5,6 +5,14 @@
 # Required
 version: 2
 
+build:
+  os: ubuntu-20.04
+  tools:
+    python: "3.7"
+  apt_packages:
+    - plantuml
+    - swig
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/conf.py
@@ -18,6 +26,5 @@ formats: all
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.7
   install:
-    - requirements: rtd_requirements.txt
+    - requirements: rtd_requirements.txt