The PEP8 suggest to use join() instead of + operator for string concatenation.

Change-Id: I35325cc73845a450f5c8ce40b7594e6d57ad6f45
Reviewed-on: https://gerrit.libreoffice.org/32504Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>