readthedocs: build with Python 3.6

The readthedocs build is failing because we do not support Python 3.5 anymore.
Bump the Python version in the configuration.

Fixes: 1b11f28 ("configure: Bump the minimum required Python version to 3.6", 2020-10-02)
Signed-off-by: Paolo Bonzini <[email protected]>
Message-id: [email protected]
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>

Author: bonzini

.readthedocs.yml

@@ -17,4 +17,4 @@ formats: all
 # we require for other Python in our codebase (our conf.py
 # enforces this, and some code needs it.)
 python:
-  version: 3.5
+  version: 3.6