Don't attach a finalizer to pynothing

Recently, when exiting Julia after using PyCall, I often get several messages like:

```
Exception SystemError: 'NULL result without error in PyObject_Call' in <built-in function weakref_callback> ignored
```

This appears to happen because Julia calls the finalizers for all Python objects at exit as of JuliaLang/julia@4796390d20a3d323de1e503e8b5f3fb0905e829e, and `pynothing` has a finalizer attached to it that sets its pointer to `C_NULL` and can be called before `weakref_callback`. This patch prevents us from attaching a finalizer to `pynothing`.