use .seek(0) instead of reset() for compatibility

Since I don't find docs for the .reset() method, I lookup up the source
code, and it should work just the same as .seek(0).

For reference:
https://github.com/python/cpython/blob/8eeb7e9122109f5cc71c22047d5cdd312ca770a0/Modules/cStringIO.c#L281