py/stream.h: Add MP_STREAM_POLL_NVAL constant.
#define MP_STREAM_POLL_WR (0x0004)
#define MP_STREAM_POLL_ERR (0x0008)
#define MP_STREAM_POLL_HUP (0x0010)
+#define MP_STREAM_POLL_NVAL (0x0020)
// Argument structure for MP_STREAM_SEEK
struct mp_stream_seek_t {
