Appends a data function to the data_files list that will generate __svn_version__.py file to the current package directory.
Generate package __svn_version__.py file from SVN revision number, it will be removed after python exits but will be available when sdist, etc commands are executed.
Notes
If __svn_version__.py existed before, nothing is done.
This is intended for working with source directories that are in an SVN repository.