pylibfdt: use python3 shebang

The default Python version for pylibfdt is already Python 3 but if
called without specifiying an interpreter, the setup.py script gets
called with Python 2.

It's of course still possible to call setup.py with python2 directly.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Message-Id: <20190907152530.25102-1-luca@z3ntu.xyz>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>