doc: expand release note fragments

newsfragments/4355.added.md

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+* Added an `ffi::compat` namespace to store compatibility shims for C API
+  functions added in recent versions of Python.
+
+* Added bindings for `PyDict_GetItemRef` on Python 3.13 and newer. Also added
+  `ffi::compat::PyDict_GetItemRef` which re-exports the FFI binding on Python
+  3.13 or newer and defines a compatibility version on older versions of
+  Python. This function is inherently safer to use than `PyDict_GetItem` and has
+  an API that is easier to use than `PyDict_GetItemWithError`. It returns a
+  strong reference to value, as opposed to the two older functions which return
+  a possibly unsafe borrowed reference.