ERP5Type/patches: prepare for removal of Products.DCWorkflowGraph

Supports the case where Products.DCWorkflowGraph is not present.
Even though we are removing Products.DCWorkflowGraph from the
software release, we don't remove this monkey patch yet, because
this monkey patch also fixed a severe security issue. We keep the
patch for the cases where a recent ERP5 runs on an old SlapOS where
the product is still there.

This change just moves the existing code in a try/except ImportError
block