This reverts commit 6135f41f49092270bdaff614bbd25aa7c0c62a41.

For some reason, the order seemed to be stable on python2

We were importing from an old alias in App.class_init, but this module
has been removed from Zope5

some conversion (TextDocument) depend on the content type, so it's
necessary to first update the content type before trying to convert to
base format