Modify DTML templates likewise Zope4 removal commit:
  commit 1df76c1ea8911d7d9249eb6beab1fafd4ed5803c
  Author: Hanno Schlichting <hanno@hannosch.eu>
  Date:   Sun Jul 3 14:19:26 2011 +0000
      Removed `bobobase_modification_time` from `Persistence.Persistent`, you can use `DateTime(object._p_mtime)` instead.

This fixes failure on test_PROPFIND_on_document (testWebDavSupport).