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As for dbm.dumb, it was an attempt to make _Database._commit() working at the shutdown stage (it is indirectly called from __del__()). See issue723231. Although in Python 2 __builtin__.open != io.open, and in Python 3.0 the code already uses io.open directly.
I afraid that this code is not reliable (and never was reliable), because modules os and io can be cleared before calling it, so os.unlink and io.open would not work. But this is a different issue.