py/scheduler: Only run scheduler callbacks queued before run started.

Without this change, a scheduler callback which itself queues a new
callback will have that callback executed as part of the same scheduler
run. Where a callback may re-queue itself, this can lead to an infinite
loop.

With this change, each call to mp_handle_pending() will only service the
callbacks which were queued when the scheduler pass started - any callbacks
added during the run are serviced on the next mp_handle_pending().

This does mean some interrupts may have higher latency (as callback is
deferred until next scheduler run), but the worst-case latency should stay
very similar.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

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