updates to the (Browser) Python coding standard and a 2->3 migration doc

[Dirk Pranke]

Hi all,

Now that the 2->3 migration for Browser has some real momentum and it's (mostly) safe to start writing Python3-only code, I thought it'd be good to update the style guide and write up at least a short overview doc on things to watch out for in the migration.

Please take a look at https://crrev.com/c/2893344 if you're interested and review it / leave me feedback.

Thanks!

-- Dirk