'Refills cstringio_buf.
Returns the currently used buffer (which can but need not be the same as
the old cstringio_buf). partialread is what the C code has read from the
buffer, and should be inserted into the buffer before any more reads.  The
return value must be a new, not borrowed reference.  Something along the
lines of self._buf should be fine.
If reqlen bytes can\'t be read, throw EOFError.'

def cstringio_refill(self, partialread, reqlen):

    pass
