try fix for readthedocs master [ci skip]

Author: afrendeiro

.readthedocs.yml

@@ -18,6 +18,10 @@ formats: all
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.7
-  install:
-    - requirements: requirements/requirements.docs.txt
+   version: 3.7
+   system_packages: False
+   install:
+      - method: pip
+        path: .
+        extra_requirements:
+           - docs