support/testing: switch to Python 3 only

Python 2.7 will not be maintained past 2020.

Many scripts on the tree are used during the build and should keep
Python 2 compatibility for a while.
This is not the case for the runtime test infra. It's meant to be run in
modern distros only, so it can safely switch to support Python 3 only.

An advantage of this approach is to have less scenarios to test in.
Otherwise every change to the test infra or runtime tests would need to
be tested against both versions of the interpreter, increasing the
effort of the developers, to ensure the compatibility to Python 2 was
not broken.

In order to accomplish the change to Python 3:
 - change the shebang for run-tests;
 - use Python 3 urllib as a drop-in replacement for Python 2 urllib2;
 - when writing the downloaded binary files, explicitly open the output
   file as binary;
 - when subprocess is used to retrieve the text output from commands,
   explicitly ask for text output. For this, use 'universal_newlines'
   because 'text' was added only on Python 3.7;
 - when pexpect is used to retrieve the text output from qemu or git,
   explicitly ask for text output using 'encoding';
 - the code using csv currently follows the example in the documentation
   for the Python 2 module, change it to follow the example in the
   documentation for the Python 3 module;
 - fix the relative import for test_git.py to be Python 3 compliant.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Tested-by: Romain Naour <romain.naour@smile.fr>
Tested-by: Nicolas Carrier <nicolas.carrier@orolia.com>
Signed-off-by: Nicolas Carrier <nicolas.carrier@orolia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>