Port to Python 3

[Aniket21mathur]

Tested with toolkit-gtk3 v0.114.

[quozl]

Thanks for 940233b. Comments;

• you've done flake8 changes mixed with other changes; this make reviewing much harder, please try to do these in a different pull request next time,
• when wrapping argument lists, please do it intelligently, allowing more than one argument per line, as the style you've chosen unnecessarily consumes lines, (limits comprehension of source code when a new developer reads the code in an editor).
• you've done only some flake8 changes; python3 -m flake8 . still gives 3 hits.