Force setup.sh to use python2

[Adrenocortico]

Short Description:

Force setup.sh to use python2

[mention-bot]

@Adrenocortico, thanks for your PR! By analyzing the annotation information on this pull request, we identified @simonsmh, @kestel and @nivong to be potential reviewers

[bruvv]

#3662
By forcing the script to use a python2 executable you're restricting the way people can install Python. Python compiles by default to python executable, so we should use it. A simple code can be added to bot's initialization to identify the Python version and exit with a warning is on Python 3.