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The problem is different. Instead, _PyString_Resize
complains that the new buffersize of the string is negative.
This in turn happens because the string manages to get
larger >2GB, which in turn happens because buffersize is
size_t, yet _PyString_Resize expects int.

I don't know how Linux manages to allocate such a large
string without thrashing.

There is a minor confusion with stat() as well:
new_buffersize tries to find out how much bytes are left to
the end of the file. In the case of /dev/zero, both fstat
and lseek are "lying" by returning 0. As lseek returns 0,
ftell is invoked and returns non-zero. Then, newbuffer does
not trust the values, and just adds BIGCHUNK.