Sometimes class name of an object changes, but to support loading
previously-saved objects, the old class name has to be also supported.

For example wendelin.core has ZBlk0 class, but historically used just
"ZBlk" name for it:

https://lab.nexedi.com/nexedi/wendelin.core/blob/v0.12-6-g318efce/bigfile/file_zodb.py#L377

Both class names have to be supported on loading and resolve to to
ZBlk0-typed runtime object.