Install pylint on the build.

* Do not clutter output with files that are OK.
* Disable flake8 for now.
* Ignore some errors.
* Ignore conventions, refactors and fatals.
Fix errors in formatted strings: (E1305) and (E1205).
Drop unnecessary pass statements: (W0107).
Fix importing names from absent modules: (E0611).
Disallow mutable values as default arguments: (W0102).
Honour relative imports: (W0403).

* Make the code more self-documented.
* Ease the transistion to Python 3.