add PyDict::get_item_with_error for PyPy

[davidhewitt]

Looks like PyPy implemented PyDict_GetItemWithError a couple of years ago in 7.2.0

While correcting I couldn't resist adjusting our wrapper to avoid checking the global exception state on a successful return, and also to use PyErr::take to only access the global exception state once on error.

[davidhewitt]

Thanks for the great feedback - I've split this out into two commits. The second commit rebuilds both get_item and get_item_with_error to use a non-generic inner, avoid the pointer-to-temporary, and use PyObject::from_borrowed_ptr_or_opt.