utils/scanpypi: use python3 explicitly

utils/scanpypi: use python3 explicitly

scanpypi is python3 compatible. In addition, it executes the setup.py
of Python modules to extract the relevant information. Since these are
more and more commonly using python3 constructs, using "python" to run
scanpypi causes problems on systems that have python2 installed as
python, when trying to parse setup.py scripts with python3 constructs.

Fixes part of #13516.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>