- Use AVIOContext instead of ByteIOContext
- Use avformat_open_input instead of av_open_input_stream
- Use avio_alloc_context instead of init_put_byte, allocate context on the heap
- Use AVIOContext.seekable

print avformat_open_input errors

avformat_open_input is the most recent and is documented as available since
libavformat 53.2.0 (other changes happened in 52.105)
avformat_open_input was introduced in 52.94 in FFmpeg