_multiprocessing leaks references when initialised multiple times

By converting `_multiprocessing` to heap types, the module can be used in subinterpreters and does not leak memory when Python finalizes or when it is initialized multiple times. The converted heap types are not performance critical.

Current Memory leak at exit:
```console
[170 refs, 71 blocks]
```

See PR https://github.com/python/cpython/pull/94336