debtags rewritten in python3

In my long quest towards closing #540218, I have uploaded a new libept to experimental. Then I tried to build debtags on a sid+experimental chroot and the result runs but has libc's free() print existential warnings about whatevers.

At a quick glance, there are now things around like a new libapt, gcc 5 with ABI changes, and who knows what else. I figured how much time it'd take me to debug something like that, and I've used that time to rewrite debtags in python3. It took 8 hours, 5 of pleasant programming and the usual tax of another 3 of utter frustration packaging the results. I guess I gained over the risk of spending an unspecified amount of hours of just pure frustration.

So from now on debtags is going to be a pure python3 package, with dependencies on only python3-apt and python3-debian. 700 lines of python instead of several C++ files built on 4 layers of libraries. Hopefully, this is the last of the big headaches I get from hacking on this package. Also, one less package using libept.