Update flake8 to 3.8.3 and drop python2 support.
- Add --extend-exclude option to add to --exclude without overwriting
- Move argument parsing from optparse to argparse
- Resolve configuration files relative to cwd instead of common prefix of passed filenames
- # noqa comments now apply to all of the lines in an explicit \ continuation or in a line continued by a multi-line string
- Improve performance by eliminating unncessary sort
- Improve messaging of --jobs argument by utilizing argparse