vdirsyncer release tarball does not have contrib files

[chenrui333]

Per this build source change, I found that the contrib files are not packaged into the pypi release artifact.

Error: An exception occurred within a child process:
  Errno::ENOENT: No such file or directory - contrib/vdirsyncer.plist

[WhyNotHugo]

contrib will be included in the next release's tarballs.