Don't include spurious files in the installed package

[PeterJCLaw]

Thanks for this package, it's handy to keep codebases clean.

I appreciate the desire to include supporting files such as the LICENSE and test_linter.py in the source distribution, however it would be great if these could be excluded from the installed package.

Currently if you pip install flake8-debugger into a clean virtualenv these files are installed into the site-packages alongside the code. The license should be installed as part of the package metadata (i.e: in the .dist-info directory) and the test file should probably not be installed at all. Its presence as a top-level importable module is likely to be surprising and have side effects users aren't expecting.