* Switch to package to Python 3:

    - Replaced python- with python3- in debian/control.
    - Removed all python2 (>= 2.7) by a ${python3:Depends}.
    - Replaced python2 by python3 in debian/rules.
    - Replaced dh_python2 stuff by dh_python3 in debian/rules.
    - Patched piupartslib/conf.py, piupartslib/dependencyparser.py,
      piupartslib/packagesdb.py, piupartslib/pkgsummary.py, tests/unittests.py
      so that they are Python 3 compatible.
    - Removed python-lzma (build-)depends as it's in standard in Py 3.
    - Add python3-six as (build-)depends.
    - Fix Makefile to use python3 and not python2.
    - d/piuparts-common.install: switch to python 3.
    - d/rules: remove all __pycache__ folder before proceeding to install.
    - Add --shebang=/usr/bin/python3 when calling dh_python3.
    - Do not use -N piuparts-master -N piuparts-slave when calling dh_python3
      for the 2nd time, so that it correctly adds the interpreter as depends.
    - Lintian overrides "python-script-but-no-python-dep": that's ok, because
      piuparts-common already has the dependency.