This allows implementing `FromPyObject` for `Vec<T>`.
It also means that `obj.extract::<Cow<str>>()` won't be supported, but we
have `PyString::extract()` for that now.
replace PyPtr<'p, PyObject<'p>> with PyObject<'p>
Borrowed python pointers stay &PyObject<'p>.
Note: this means the internal representation of borrowed python pointers
changes from "*mut PyObject" to "&*mut PyObject".
Daniel Grunwald