install graphviz on RTD, make RTD fail on warnings (#152)

Author: svenevs

.readthedocs.yml

@@ -10,18 +10,13 @@ build:
   os: ubuntu-20.04
   tools:
     python: "3.9"
-    # You can also specify other tool versions:
-    # nodejs: "16"
-    # rust: "1.55"
-    # golang: "1.17"
+  apt_packages:
+    - graphviz
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
    configuration: docs/conf.py
-
-# If using Sphinx, optionally build your docs in additional formats such as PDF
-# formats:
-#    - pdf
+   fail_on_warning: true
 
 # Optionally declare the Python requirements required to build your docs
 python: