Use spaces instead of tabs

This is a Request For Comments, based on @shoghicp's commit `ebd771a9fa3f848bfd7294ac8c38db0a1310adf9` that was pushed without any community discussion. The offending commit has been reverted from `master`. Please make sure that any changes affecting CONTRIBUTION, code guidelines and project-wide changes have first been accepted through an RFC process with a simple majority vote.

I am not sure about the reason behind this change, so it would be great if @shoghcp could elaborate on the reasoning behind this change.