Replace tabs with spaces in .py files

Python coding convention is to use four spaces for indentation, not tabs.
We have to stick with one method as Python 3 disallows the mixture of
tabs and spaces for indentation.

I have changed all the tab indentations I could find with four spaces.

Sublime Text users, please check indentation settings here:
https://www.sublimetext.com/docs/3/indentation.html