python3: use built-in distutils for ptest, rather than setuptools' 'fork'

The setuptools version is now used by default when setuptools is installed,
but it  is incompatible with some of python's own tests.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>