Pybind incorrectly deregisters instances (by type comparison), yields errors when C++ recycles memory

I ran into an interesting (read: Heisen) bug that occurred when I had code that looked like this:

obj = m.UniquePtrHeld(1)
m.unique_ptr_terminal(obj)  # Destroys the value held by `obj`
obj = m.UniquePtrHeld(1)

After C++ destroyed the first instance, the second instance was allocated into the same address, when casting the instance back, pybind registered a new instance (in get_internals().registered_instances) when the same ptr-value, but a different instance.
In deregister_instance_impl, it finds all values that match the given ptr, and only checks if the Py_TYPE is the same.

The fix in this case is just to compare instances (if there wasn't a strong reason for getting the type). An example change:
https://github.com/EricCousineau-TRI/pybind11/blob/feature/unique_ptr_arg_pr1-wip/include/pybind11/detail/class.h#L225

Even though I encountered this for my unique_ptr PRs, I believe it could still be an issue when returning bare pointers, say:

// c++
struct MyType { int value; }
...
m.def("do_something", [](MyType *in) {
  delete in;
  return new MyType{10};
});

# python
obj = m.MyType(10)
obj = do_something(obj);

Start in include/pybind11/detail/class.h around line 225, then trace deregister_instance_impl and the registered_instances lookup. Reproduce the pointer-address reuse examples with unique_ptr and bare pointers, and verify that deregistration distinguishes the original instance from a new object at the same address without producing errors.