pylibfdt: fix compatibility with python2

[jpalus]

c001fc01a43e ("pylibfdt: fix swig build in install") introduced python3-only super() invocation that fails with python2

File "setup.py", line 48, in run return super().run() TypeError: super() takes at least 1 argument (0 given)

while setup.py still claims to be compatible with both python versions:

While Python 3 is the default, it's also possible to invoke this setup.py script with Python 2.

use legacy syntax that works in all python versions

Fixes: c001fc01a43e ("pylibfdt: fix swig build in install")