Changes

Migrate to python 3

Given that on most systems Python is still linked to the Python 2.7,
switch explicitly to using Python 3.

Python 2.7 was discontinued on January 1, 2020: [1].

Moreover, there is PEP recommending to use python3 in the shebang: [2].

[1] https://www.python.org/doc/sunset-python-2
[2] https://www.python.org/dev/peps/pep-0394

Change-Id: Idd3ed567b0384c6ff0a6121ad736f8af4c0f85e0