bpo-40545: Export _PyErr_GetTopmostException() function (GH-19978)

Declare _PyErr_GetTopmostException() with PyAPI_FUNC() to properly export the function in the C API. The function remains private ("_Py") prefix. Co-Authored-By: Julien Danjou <julien@danjou.info>

Author: vstinner

Include/cpython/pyerrors.h

@@ -75,7 +75,7 @@ typedef PyOSErrorObject PyWindowsErrorObject;
 /* Error handling definitions */
 
 PyAPI_FUNC(void) _PyErr_SetKeyError(PyObject *);
-_PyErr_StackItem *_PyErr_GetTopmostException(PyThreadState *tstate);
+PyAPI_FUNC(_PyErr_StackItem*) _PyErr_GetTopmostException(PyThreadState *tstate);
 PyAPI_FUNC(void) _PyErr_GetExcInfo(PyThreadState *, PyObject **, PyObject **, PyObject **);
 
 /* Context manipulation (PEP 3134) */

Misc/NEWS.d/next/C API/2020-05-07-11-41-13.bpo-40545.51DzF1.rst

@@ -0,0 +1,3 @@
+Declare ``_PyErr_GetTopmostException()`` with ``PyAPI_FUNC()`` to properly
+export the function in the C API. The function remains private (``_Py``)
+prefix.