This is a [breaking-change] and makes the trait more difficult to implement.
The usage through PyObject::extract() is unchanged.
This change allows extracting a `&str` through a temporary `Cow<str>`
without having to copy the string data from python to rust
(at least in cases where the python string is UTF-8 encoded).
This is preparation in hope I'll be able to make py_fn!()
automatically extract the function arguments.
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