Thanks a lot for the code review! I'm attaching a revised version of the patch. Fixes I made:

- added check whether PyLong_AsLong returned an error
- removed "ADD_INTERNED(_blksize)" and "PyObject *_PyIO_str__blksize;" - I noticed that these are only necessary when exported by _iomodule.h, which isn't needed for _blksize ATM
- moved blksize to a place of fileio structure where it won't create unnecessary padding

I hope attaching the version 2 of the patch here is ok, if I should have attached it in the code review tool somehow, please let me know.