Should we restrict pip version to pip2?

[bcho]

Some OS use python3 / pip3 as default, but supervisor requiers python2.x . I think it's better to restrict pip executable to pip2.

[skolsuper]

Just ran into an issue with this, have edited the file locally to include the executable=pip2 parameter without issue. Obviously, it's not a good solution to have local changes so I will make a PR.

Up this issue, still necessary.

[avelis]

bump