Declare Exception as a builtin type.

This allows extending from it with a cdef class. Closes #724.

Cython/Compiler/Builtin.py

@@ -324,6 +324,7 @@ builtin_types_table = [
                                     BuiltinMethod("add",     "TO", "r", "PySet_Add"),
                                     BuiltinMethod("pop",     "T",  "O", "PySet_Pop")]),
     ("frozenset", "PyFrozenSet_Type", []),
+    ("Exception", "((PyTypeObject*)PyExc_Exception)[0]", []),
 ]
 
 
@@ -377,6 +378,8 @@ def init_builtin_types():
             objstruct_cname = 'PySetObject'
         elif name == 'bool':
             objstruct_cname = None
+        elif name == 'Exception':
+            objstruct_cname = "PyBaseExceptionObject"
         else:
             objstruct_cname = 'Py%sObject' % name.capitalize()
         the_type = builtin_scope.declare_builtin_type(name, cname, utility, objstruct_cname)

tests/run/builtin_type_inheritance_T608.pyx

@@ -114,3 +114,14 @@ cdef class MyDict(dict):
     True
     """
     cdef readonly object attr
+
+cdef class MyException(Exception):
+    """
+    >>> raise MyException(3)
+    Traceback (most recent call last):
+    ...
+    MyException: 3
+    """
+    cdef readonly int value
+    def __cinit__(self, value):
+        self.value = value