D6556: cleanup: use named constants for second arg to .seek()

[Yuya Nishihara]

> @@ -16,6 +16,7 @@
>  import collections
>  import contextlib
>  import errno
> +import io
>  import os
>  import struct
>  import zlib
> @@ -2306,7 +2307,7 @@
>  
>          try:
>              with self._datafp() as f:
> -                f.seek(0, 2)
> +                f.seek(0, io.SEEK_END)

Nit: can be `os.SEEK_END` since we don't use the (new) io module on Python 2.