Try to fix readthedocs (#539)

Author: jan-janssen

.readthedocs.yml

@@ -21,6 +21,7 @@ build:
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   builder: html
+  configuration: docs/conf.py
 
 # Optionally build your docs in additional formats such as PDF and ePub
 formats: []