micropython: Change mp_task's priority back to 1

Our fork has bumped the priority up by one, but it failed to also adjust the priorities of other tasks that may get spawned by micropython (such as RMT, I2S or _thread). The change seems to have been made to make its priority higher than the graphics task's, but the graphics task is pinned to another core these days (the blit task remains unpinned at priority 2).

Change the mp_task's priority back to the one used by upstream.