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

Fixes: 1b11f28d ("configure: Bump the minimum required Python version to 3.6", 2020-10-02)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20201005150122.446472-1-pbonzini@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>