python3-poetry-core: drop python3-six from RDEPENDS

python3-poetry-core: drop python3-six from RDEPENDS

Looking at the history, python3-six was removed as a dependency
in the poetry.lock file in v1.5.2

Even before v1.5.2 and until now (v1.9.1) there is no code in
the package which imports the six module. So it can be safely
dropped from the recipe.

Signed-off-by: Guðni Már Gilbert <gudni.m.g@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>