Using spaces instead of tabs for indents

I found several files which were using spaces instead of tabs. Even files where both was mixed.
I read the DEVELOP file and I'm aware of the style guide to use only tabs.

Some reasons why it would be nice to switch to spaces:
 - it's PEP8 style and for this more common
 - most python editors are using spaces by default
 - spaces have the same length in every editor no matter what is set in yours

 I just went through the python files. So other files may still concern tabs and mixed tabs.