core: Fix unicode bug on ZODB History view.

Some objects, in our case BTrees.Length.Length in a ZODB connection
have a __str__ method that returns unicode on python2:

u'<BTrees.Length.Length object at 0x7f850932e0d0 oid 0x1e334 in <Connection at 7f854bc0f190>>'

They cause an unicode error in the history view when they are
concatenated together with other str (encoded as UTF-8) properties,
this can be observed when using history view with a "folderish"
document (but not with a File as in test_ZODBHistoryBinaryData).

To prevent this issue, we use the fact that ''.format unlike '' %
seem to apply a str() on arguments and use it instead.

Co-authored-by: Yusei Tahara <yusei@nexedi.com>