Fix code to comply with stricter code styleguides

Created by: cburchert

I locally use a stricter version of flake8. I fixed all the errors I get, which are not covered in the projects flake8 settings.

The three commits include:

1. import ordering by Google Styleguide (https://google.github.io/styleguide/pyguide.html#Imports_formatting).

• Imports are in three groups: stdlib, external and internal
• no relative imports
• imports alphabetically (case insensitive)

2. Variables may not shadow builtins

3. Various errors which somehow slipped earlier, mostly wrong intendation.