test_argcomplete should not call python directly

[scarabeusiv]

on the most distros it points to py2 or it is not really deterministic.

The call itself is following:
fp.write('COMP_WORDBREAKS="$COMP_WORDBREAKS" python -m pytest 8>&1 9>&2')

I suppose it should use sys.executable or at least python3 as the py2 is not compatible anyway.

[asottile]

sys.executable would be best, would you like to submit a patch? thanks!

(it mostly works right now because it runs in a virtualenv)