I'm distributing command line tools by pypi, and the package also installs shell completion file.
The default shell completion install location is sometimes platform specific, so the code cannot be so simple. 
I think it is good to have a function which can be used easily when distributing this kind of package. I think the function is similar with ``distutils.sysconfig.get_python_lib()``.

Please see attached file. It contains a function I'm thinking to add now.