pyio seek's return type should be int64_t

Created by: xxr3376

PyAV will throw OverflowError: value too large to convert to int when opening a python file object larger than 2GB, but open by filename is ok. I found that pyio_seek_gil should return int64_t instead of int for opening a large file.

Sorry for wrong insert about setup.py, this pull request has reseted that commit.