stream_readall: This can be long-running, run background tasks

While finding sources of clicks and buzzes in nrf i2sout, I identified
this site as one which could be long running.  Reproducer code was to
play a 22.05kHz sample and repeatedly print `os.listdir('')`

Author: jepler

py/stream.c

@@ -341,6 +341,7 @@ STATIC mp_obj_t stream_readall(mp_obj_t self_in) {
             p = vstr_extend(&vstr, DEFAULT_BUFFER_SIZE);
             current_read = DEFAULT_BUFFER_SIZE;
         }
+        RUN_BACKGROUND_TASKS;
     }
 
     vstr.len = total_size;