runtests: Allow usage of virtual environments
    
    Currently the python3 interpreter location is hardcoded in the shebang.
    This is problematic when running the tests in distros that place it
    elsewhere or when leveraging virtualenvs to run the tests. With the
    proposed change we just rely on python3 being in the environment.
    
    Merges: #3789
    
    Signed-off-by: Antoni Segura Puimedon <celebdor@gmail.com>