Raising dbm.dumb.error is behavior change. It would be safer not apply this part in maintained releases.

If add sanity checks in 3.5, note that following line "key = key.encode('Latin-1')" can raise an exception too (AttributeError or UnicodeEncodeError). And incorrect data can cause an error later in __getitem__ if pos_and_siz_pair is not a pair of two integers.

I think it is worth to split this issue on two issues and fix only security issue here.