python3-manifest: turtle: new package

Extend the manifest with definitions for the turtle package from the
Python standard library. Currently this package cannot be imported at
run-time because python3-misc doesn't pull the required dependencies.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>