Update shebang to python3

[ruffsl]

It seems the debian install is broken without this, e.g:

$ rosdep
Traceback (most recent call last):
  File "/usr/bin/rosdep", line 3, in <module>
    from rosdep2.main import rosdep_main
ImportError: No module named rosdep2.main

[nuclearsandwich]

I'm pretty sure that rewriting the shebang is done during the python3 deb release in a programmatic fashion and that changing it here would have unintended affects for those installing this script via other methods.

[ruffsl]

Ok, never mind this PR then.

[nuclearsandwich]

I've not substantiated the theory but my hypothesis is that the incorrect line endings present when creating the 0.16.0 release interfered with the shebang modification during release. 0.16.1 which was generated from a fixed repository clone appears to work on the bootstrap repository and is moving out to the main repositories now.