package/dbus-python: fix build with python 3.8

package/dbus-python: fix build with python 3.8

test/import-repeatedly.c uses an embedded python interpreter
and PYTHON_EXTRA_LIBS is used only for building this test case,
so set PYTHON_EXTRA_LIBS with `python3-config --libs --embed`
to build this test case and don't link the module with libpython3.so.

Fixes:
 - http://autobuild.buildroot.org/results/b30/b308eeb5c5d95ab9f1dbfc19f9183f2ba3ba0ce3/
 - http://autobuild.buildroot.org/results/0dd/0dd9203f859b97ee5a3b6358644c26f8ab784ed8/
   and many similar failures.

Signed-off-by: Grzegorz Blach <grzegorz@blach.pl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>