python, python3: fix to ensure libpython is stripped

The python and python3 builds mark libpython as read-only which
prevents it from being stripped out correctly for the target.

Signed-off-by: Przemyslaw Wrzos <przemyslaw.wrzos@calyptech.com>
Acked-by: Thomas De Schampheleire <thomas.de_schampheleire@alcatel-lucent.com>
Tested-by: Thomas De Schampheleire <thomas.de_schampheleire@alcatel-lucent.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>