flake8 + initial fixes

[diogobaeder]

This pull request introduces flake8, and does some initial fixes for the tool. Mostly, they are:

• Linefeed additions (to have 2 LFs between top-level symbols);
• Removal of unused variables (note that this allowed us to remove some of the imports as they became useless, like, for example pub_date = du_parser.parse(article["pub_date"]).date(), which required du_parser from dateutil);
• Imports reordered, separating with linefeed between:
  • Internal modules
  • 3rd party
  • Project-related
• Stick to 4-spaces for indentation

[oyiptong]

you're on a roll :-)

[diogobaeder]

Thanks, man! My pleasure! :-)