gr-baz should switch to Python 3

[rhertzog]

Python 2.x will no longer be supported by their upstream developers in 2020. It is thus time to switch to Python 3.

gnuradio in Debian testing already switched to Python 3. It would be nice if gr-baz could follow the move as well. But the repository seems rather inactive lately. Is that software still maintained ?

In Kali we still have the package but we already apply the patch from #56 and another patch fixing the detection of the Gnuradio package. If the package can't be updated to work with Python 3, it will likely be removed at some point (tracked in https://gitlab.com/kalilinux/packages/gr-baz/issues/1).

Thank you for your work on gr-baz!