Porting PyClean to python3

[Julien-Elie]

This PR is to apply after #7. It is made of 3 commits:

• Copy dictionary keys when modified inside a loop as Python 3.x no longer copies the dictionary when getting the keys, and therefore the loop fails with an exception because the dictionary is changed inside the loop.
• Properly handle memoryview objects, encoding and decoding them when appropriate. Use UTF-8 when translating from or to strings, as well as when logging articles.
• Use a portable syntax for Exception.