py/stream: Use uPy errno instead of system's for non-blocking check.

This is a more consistent use of errno codes.  For example, it may be that
a stream returns MP_EAGAIN but the mp_is_nonblocking_error() macro doesn't
catch this value because it checks for EAGAIN instead (which may have a
different value than MP_EAGAIN when MICROPY_USE_INTERNAL_ERRNO is enabled).

py/stream.h

@@ -107,7 +107,7 @@ int mp_stream_posix_fsync(mp_obj_t stream);
 #endif
 
 #if MICROPY_STREAMS_NON_BLOCK
-#define mp_is_nonblocking_error(errno) ((errno) == EAGAIN || (errno) == EWOULDBLOCK)
+#define mp_is_nonblocking_error(errno) ((errno) == MP_EAGAIN || (errno) == MP_EWOULDBLOCK)
 #else
 #define mp_is_nonblocking_error(errno) (0)
 #endif