adding new getter for type obj

[Cheukting]

Adding new getter that works like __mro__ and __bases__ in Python.

closes #4192

[Cheukting]

Got it, seems we will roll back to the unsafe way to do it, right? @davidhewitt I will update this PR soon.

[davidhewitt]

Failure looks genuine:

  error: the item `FfiPtrExt` is imported redundantly
     --> src/types/typeobject.rs:163:17
      |
  3   | use crate::ffi_ptr_ext::FfiPtrExt;
      |     ----------------------------- the item `FfiPtrExt` is already imported here
  ...
  163 |             use crate::ffi_ptr_ext::FfiPtrExt;
      |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
      = note: `-D unused-imports` implied by `-D warnings`
  
  error: could not compile `pyo3` due to previous error

... seems like some of the FfiPtrExtr imports lower down in this file can probably just be removed now that we've added an import up the top.

[Hooksie]

Thanks for implementing!

[Cheukting]

@davidhewitt thanks! Will have a look at the unused imports

[Hooksie]

Adding some really small feedback on the expect message. Feel free to disregard if it's not important

[Cheukting]

Thank you @Hooksie