add PyDictProxy, see #221

[jruiss]

The mappingproxy functions are defined as static in https://github.com/python/cpython/blob/3.7/Objects/descrobject.c#L806 and tbh, dunno what's the point is with _Py_IDENTIFIER(keys) and _PyObject_CallMethodId for example in mappingproxy_keys so went directly to the source in abstracts by checking _Py_IDENTIFIER(keys) in https://github.com/python/cpython/blob/088b63ea7a8331a3e34bc93c3b873c60354b4fad/Tools/c-analyzer/known.tsv#L1212 etc.. https://github.com/python/cpython/blob/4a21e57fe55076c77b0ee454e1994ca544d09dc0/Objects/abstract.c#L2303

Anyway, everything works as expected and so on.

Usage

let attrs = pyobject.getattr(py, "__dict__").unwrap()
            .cast_into::<PyDictProxy>(py).unwrap()
            
let items = attrs.items(py).cast_into::<PySequence>(py).unwrap();

[markbt]

Thanks for the contribution!

I think I understand this, but could you add some more comments in the commit message (and ideally improve the commit title, too, so that it fits in more with the other commits in the repo).

Python 2.7 is still a supported platform for now (e.g. Mercurial uses it). Can you make the corresponding changes in python27-sys, please?

[markbt]

Please improve the commit message, maybe add docs and a test case, and make it work on Python 2.7 also.

Thanks for the contribution!