recipetool/create_buildsys_python: prefix created recipes with python3-

recipetool/create_buildsys_python: prefix created recipes with python3-

By convention, all python recipes start with "python3-" so update
create_buildsys_python to do this

This rule doesn't apply for packages already starting with "python"

Update recipetool's selftest accordingly

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>