Replace python-debian dependency with statically provided code

[mxmehl]

We want to get rid of python-debian as a dependency because it creates a number of problems on non-Debian/apt environments, and is generally containing a lot of code we don't need.

Instead, let's export the relevant code and ship it with reuse-tool directly.

Would fix #425 and #427, and probably make fixing issues like #454, #283, #191 and #163 much easier.

[llimeht]

Writing on behalf of the python-debian maintainer team:

We want python-debian to be portable and widely usable. We definitely don't want people to feel the need to fork it, particularly not for simple things like portability of Python code.

We've got a stack of portability fixes ready for the next release, with all tests now passing under Debian, Alpine, a stripped-down-Debian, and Windows. Are there other problematic hosts we should test against? We're happy to hear from you if you find other bugs or if you have other enhancements for python-debian.

[Tachi107]

As this hasn't been mentioned before, I think that nexB/debian-inspector might be an interesting alternative in the case that python-debian still doesn't suit your needs. (Cc: @pombredanne)