[mypyc] Make for loop over list memory-safe on free-threaded builds (#21686)

Don't access list items directly but use CPython C API functions for
item access. Also, reading the list length should use an atomic relaxed
memory order read, but changing this is lower priority and won't be
fixed until later. There is no change on non-free-threaded Python
builds.

Work on mypyc/mypyc#1202.