I don't much like the use of Py_SSIZE_T and Py_SAFE_DOWNCAST here:  the dtoa.c code knows almost nothing about Python.h (aside from right at the top), and I'd like to keep it that way if possible.

And in fact I'd say that we *shouldn't* be silencing these warnings; rather, we should take them seriously.  It looks to me as though it is possible for that conversion to overflow.

I'll try to take a look sometime soon.  Adding 2.7 and 3.3, since the bug is present there, too.