add failing Py3 exception test

tests/bugs.txt

@@ -12,6 +12,7 @@ for_from_pyvar_loop_T601
 temp_sideeffects_T654    # not really a bug, Cython warns about it
 slice2_T636
 inherited_final_method
+tryfinallychaining        # also see FIXME in "yield_from_pep380" test
 
 # CPython regression tests that don't current work:
 pyregr.test_signal

tests/run/tryfinallychaining.pyx

+# mode: run
+# tag: exceptions, tryfinally
+
+import sys
+IS_PY3 = sys.version_info[0] >= 3
+
+
+def test_finally_c():
+    """
+    >>> def test_finally_py():
+    ...     try:
+    ...         raise AttributeError()
+    ...     finally:
+    ...         raise KeyError()
+
+    >>> try:
+    ...     test_finally_py()
+    ... except KeyError:
+    ...     print(sys.exc_info()[0] is KeyError or sys.exc_info()[0])
+    ...     if IS_PY3:
+    ...         print(isinstance(sys.exc_info()[1].__context__, AttributeError)
+    ...               or sys.exc_info()[1].__context__)
+    ...     else:
+    ...         print(True)
+    True
+    True
+
+    >>> try:
+    ...     test_finally_c()
+    ... except KeyError:
+    ...     print(sys.exc_info()[0] is KeyError or sys.exc_info()[0])
+    ...     if IS_PY3:
+    ...         print(isinstance(sys.exc_info()[1].__context__, AttributeError)
+    ...               or sys.exc_info()[1].__context__)
+    ...     else:
+    ...         print(True)
+    True
+    True
+    """
+    try:
+        raise AttributeError()
+    finally:
+        raise KeyError()