flake8: check for mixed tabs/spaces

The main change here is to flag mixed tabs/spaces (E101) in the flake8 checking as some of this code is not valid in Python3 and in general is very confusing to look at. Make all code that is being checked in our build compliant, i.e. the whitespace changes in this MR are intentional.

Some other changes to our default flake8 configuration:

• Revert running with a single process as this was not the cause for timeouts (ATLINFR-3046) and increases the time for local builds.
• Use --ignore instead of --extend-ignore for our default ignore list. This allows developers to use the latter to extend the ignore list at the package level if needed.