C0330 compliance (wrong indentation)

This PR fixes the C0330 error code that is raised, when wrong indentation occurs.
I made a small change to CONTRIBUTING.md: the file states now explicitly how to run pylint on the project. (Would have helped me, so I thought other new contributors might find that helpful as well.)
The change in data_uri.py is actually a C0111 fix (two characters is considered to short for a variable name). It ended up in here by accident. Let me know, if I should remove it