Update style guide to use 4 space indents for Python and some other practices

[kvas]

Background

Our style guide recommends 2 space indents for all code, including Python code. This is in contradiction to PEP8 recommendation. So we recently agreed to move to 4 spaces for Python code, while sticking to 2 spaces for other languages, just like Mozilla does.

There are also a few other unspoken rules that we normally follow but that are not in the style guide. It would shorten code review discussions and make them more objective if we add such things to the style guide.

What to change

• Change the style guide to just follow Mozilla style guide for formatting. We already do that but override indentation in our style guide. Since we choose to follow PEP8 for Python, we don't need the override.
• Specifically mention PEP8 in the Python section of the style guide.
• Add a point about string concatenation and formatting (+ to be used only for 2 strings, .join and .format preferred for more).
• Add a point about choosing between using tuples and lists (based on ​https://docs.python.org/2/tutorial/datastructures.html#tuples-and-sequences).

[abpbot]

A commit referencing this issue has landed:
​https://hg.adblockplus.org/web.adblockplus.org/rev/8ab2e83850a6