Returns contents of the given file, which path is supposed relative
    to this package.

def read_relative_file(filename, relative_to=None):
    """Returns contents of the given file, which path is supposed relative
    to this package."""
    if relative_to is None:
        relative_to = os.path.dirname(__file__)
    with open(os.path.join(os.path.dirname(relative_to), filename)) as f:
        return f.read()