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The docs of pkgutil.get_data say "The resource argument should be in the form of a relative filename, using / as the path separator. The parent directory name .. is not allowed, and nor is a rooted name (starting with a /)."
In fact (on Python 3.5 at least):
* pkgutil.get_data("logging", "/__init__.py") works, but simply chops off the first slash, returning the contents of the stdlib's logging/__init__.py.
* pkgutil.get_data("logging", "../re.py") works, returning the contents of the stdlib's re.py.
People who actually thought about the implications of get_data/zipimport/etc. can decide whether to remove this functionality or to update the docs, I'm just reporting it.
Also, it would be nice if get_data gained a "text mode" (i.e. returning str instead of bytes and with support for universal newlines).

Changing this to be a single issue about the difference between the docs and the semantics of pkgutil.
The feature request can be made into a separate issue, but there are plans to replace the API with a more stringently defined one in importlib.

I think it can even be considered a security bug. A classic path traversal. The fact that documentation falsely suggests that there's no such vulnerability is clearly not helping.
Python 2.7 is affected as well, by the way.

This can't change in Python 2.7 because of backwards-compatibility. And I would argue this isn't a serious security risk as pkgutil.get_data() typically works with string constants and values provided by the library and not user-provided values. This is basically the same as taking a value for open() and has the same risks.

I'm not sure if this warrants a separate issue, but I also notice this in the documentation:
> If the package cannot be located or loaded, or it uses a loader which does not support get_data, then None is returned. In particular, the loader for namespace packages does not support get_data.
But in reality this seems to raise a FileNotFoundError:
 >>> import pkgutil
 >>> data = pkgutil.get_data('dateutil.zoneinfo', 'dateutil-zoneinfo.tar.gz')
 >>> len(data)
 139671
 >>> data = pkgutil.get_data('dateutil.zoneinfo', 'foo-bar.tar.gz')
 FileNotFoundError: [Errno 2] No such file or directory: '/usr/lib/python3.6/site-packages/dateutil/zoneinfo/foo-bar.tar.gz'
Am I misunderstanding the documentation, or should the failure mode be corrected to specify that it raises an error?

Notice that the returning of None only has to do with searching for the *package*, not the *data file*. So I think the docs are still correct according to your example, Paul.