fix error raised by pylint 1.8

[aabadie]

Pylint 1.8 is more strict now and new type of issues are raised:

• Keyword argument before variable positional arguments list in the definition of __init__ function: fixed by ignoring the issue because it can only be cleanly fixed with Python3
• Either all return statements in a function should return an expression, or none of them should: requires a straightforward change