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The cause of the bug is that if oldtable == mp->ma_smalltable
then pure Python code can mangle with mp->ma_smalltable while
it is being walked on.  A simple fix would be to always make
a copy of the oldtable if it is mp->ma_smalltable (not only
if oldtable == newtable).

Attached a more efficient fix, which should also make dict
resizing somehow faster.  It requires yet another version
of the lookup algorithm, though.  It's a very simple version
that assumes that all items are different and the dict
contains no dummy entries.