Let's not use the warnings module

The output is all wrong:

    ./check_manifest.py:96: UserWarning: You have a wrong line in MANIFEST.in: 'recursive-exclude hehe'
    'recursive-exclude' expects <dir> <pattern1> <pattern2> ...
      warnings.warn(message)

It's clearly designed for an audience of programmers who write source
code incorrectly, not users who run command-line programs.