[mypyc] Make list get/set item more memory safe on free-threaded builds (#21683)

Use the CPython C API for list get item and set item, as direct item
access is not memory safe in the presence of race conditions on a
free-threaded Python.

Note that these operations are not necessarily atomic -- this only fixes
memory safety. Explicit synchronization is still generally expected for
correctness.

This still doesn't cover for loops over lists. I will fix them in a
follow-up PR.

Preserve existing optimized primitives on non-free-threaded builds.

Work on mypyc/mypyc#1202.