Projects: Changes to 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.
- Add `E101` (mixed tabs/spaces) to the list of selected errors.