Stackless issue #252: Update .readthedocs.yml to format version 2

Update the configuration file for readthedocs.org to the current format.
The previous format is deprecated and fails to build.

Author: Anselm Kruis

.readthedocs.yml

@@ -1,8 +1,20 @@
+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+   configuration: Doc/conf.py
+
 # Don't build any extra formats
 formats: []
 
 build:
-    image: 2.0
+   image: stable
 
+# Optionally set the version of Python and requirements required to build your docs
 python:
-    version: 3.5
+   version: 3.6