.readthedocs.yml: specify some minimum python requirements

QEMU is all about the Python 3 now so lets also hint that to
ReadTheDocs in its config file.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200206163120.31899-1-alex.bennee@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

.readthedocs.yml

+# .readthedocs.yml
+# 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: docs/conf.py
+
+# We want all the document formats
+formats: all
+
+# For consistency, we require that QEMU's Sphinx extensions
+# run with at least the same minimum version of Python that
+# we require for other Python in our codebase (our conf.py
+# enforces this, and some code needs it.)
+python:
+  version: 3.5