The Python2 PyYAML is now pointless, switch to the Python3 version.

Fixes: bcbf2794 (docker: move tests from python2 to python3)
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200322120104.21267-4-philmd@redhat.com>
Message-Id: <20200323161514.23952-10-alex.bennee@linaro.org>