py/gc: Execute finaliser code in a protected environment.

If a finaliser raises an exception then it must not propagate through the
GC sweep function.  This patch protects against such a thing by running
finaliser code via the mp_call_function_1_protected call.

This patch also adds scheduler lock/unlock calls around the finaliser
execution to further protect against any possible reentrancy issues: the
memory manager is already locked when doing a collection, but we also don't
want to allow any scheduled code to run, KeyboardInterrupts to interupt the
code, nor threads to switch.