Define _PyErr_ChainExceptions() FFI for CPython

This appears to be the only way to chain exceptions via the FFI. It
was introduced prior to CPython 3.7. It has some description of usage
in PEP 490. There was a discussion of whether it should be made stable
in BPO 44938.

newsfragments/2788.added.md

@@ -0,0 +1 @@
+Define `_PyErr_ChainExceptions()` FFI for CPython.

pyo3-ffi/src/cpython/pyerrors.rs

@@ -150,6 +150,11 @@ pub struct PyStopIterationObject {
     pub value: *mut PyObject,
 }
 
+extern "C" {
+    #[cfg(not(PyPy))]
+    pub fn _PyErr_ChainExceptions(typ: *mut PyObject, val: *mut PyObject, tb: *mut PyObject);
+}
+
 // skipped PyNameErrorObject
 // skipped PyAttributeErrorObject
 
@@ -160,8 +165,6 @@ pub struct PyStopIterationObject {
 // skipped _PyErr_GetTopmostException
 // skipped _PyErr_GetExcInfo
 
-// skipped _PyErr_ChainExceptions
-
 // skipped PyErr_SetFromErrnoWithUnicodeFilename
 
 // skipped _PyErr_FormatFromCause