scons: Switch to Python 3

Reasons:

Python 2.7 will EOL very soon [0]:
DEPRECATION: Python 2.7 will reach the end of its life on January 1st,
2020. Please upgrade your Python as Python 2.7 won't be maintained after
that date. A future version of pip will drop support for Python 2.7.
More details about Python 2 support in pip, can be found at
https://pip.pypa.io/en/latest/development/release-process/#python-2-support

SCons 4.0.0 will drop Python 2.7 Support [1]:
https://raw.githubusercontent.com/SConsProject/scons/rel_3.1.2/src/CHANGES.txt

[0]: From the SCons build output previous to this commit (i.e. with Python 2.7).
[1]: https://raw.githubusercontent.com/SConsProject/scons/rel_3.1.2/src/CHANGES.txt