Detects and interactively deactivates duplicate Apt source entries and deletes sources list files without valid enabled source entries (as requested in https://askubuntu.com/a/762815/175814).
Fix-up of another issue in v0.1.7.5
Fix issue #33.
Fixes issues #30 and #31.
Fixes issue #21 which affects Python 3.5 (Ubuntu 16.04) when running from a ZIP archive.