Remove GetPropertyValue & improve property docs

[davidhewitt]

Now that we have Clone for PyObject and Py<T>, we don't need GetPropertyValue any more, so this cleans that up.

At the same time I also:

• Added a test for Vec<PyObject> with #[pyo3(get)] (which was broken in 0.9.2, see Generic lists #903)
• Improved documentation for #[pyo3(get, set)].

This is all internal refactoring so I think this can be merged without blocking us from making another patch release for 0.10.x if necessary.

Closes #903

[kngwyu]

Thank you!
Could please add a CHANGELOG entry for this?

[davidhewitt]

CHANGELOG added 👍

[kngwyu]

Thanks!