Rewrite in Python3

Bash / POSIX Shell was getting way too limited for this kind of work. I
was mainly missing proper data structures.

lib/_dcf.py is a good start for a quite generic Debian control file
parser. If it evolves enough, it would be wise to extract it to an
independent library.
Update README
Fix packaging for new Python3-dependent aptirepo

Packaging is fairly simple but ugly. This need to be fixed at some
point.
Use the python version aptirepo-import
Fix _assert_simple
Make apt-ftparchive binary cache db relative to the rootdir
Add restricted component
Run apt-ftparchive in the repository root directory
Fix aptirepo-upload remote
Version +0.3.0