python-pygame: remove python dependency

With the dependency on BR2_PACKAGE_PYTHON it was only possible to
build the package with python2. However, python-pygame works with
python3 so enable it to be built with it.

Signed-off-by: Peter Bouda <pbouda@cidles.eu>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>