Resolve tools class path import issue
Resolve tools class path import issue

Importing tools in backend.py introduced an issue around
module vs class importing.  This change switches to relative
import of tools.

fixes xflr6/graphviz#24

Signed-off-by: Dave Walker (Daviey) <a88b7dcd1a9e3e17770bbaa6d7515b31a2d7e85d@daviey.com>

xflr6/graphviz