BUG: Fix large file support on 32 bit

In order to use files larger than 32 bit on 32 bit systems one must use 64 bit variables for all seeks. On linux this means enabling the lfs macros and using ftello/fseeko. Windows was also broken due to a conversion to Py_Ssize_t.

The added test does not need to write 5GB of data on sparse filesystems so it is enabled by default.

Also update the travis tests 32 bit to run under python3 as the bug is only triggered with that version.