update pico support for CircuitPython 8

[prubel]

the previous code (CicruitPy 7) no longer works with the latest circuit python, (8.x) due to an API change.

set the stack size using the new supervisor.runtime.next_stack_limit attribute rather than the no-longer-supported supervisor.runtime.set_next_stack_limit method

remove unused imports in code.py

note in the readme that it's been testing with 8.2 so future folks have a clue as to what's going wrong