Support throw() of string exceptions.

Lib/test/test_generators.py

@@ -1545,6 +1545,9 @@ caught ValueError (1)
 >>> g.throw(ValueError, TypeError(1))  # mismatched type, rewrapped
 caught ValueError (1)
 
+>>> g.throw(ValueError, ValueError(1), None)   # explicit None traceback
+caught ValueError (1)
+
 >>> g.throw(ValueError(1), "foo")       # bad args
 Traceback (most recent call last):
   ...
@@ -1592,8 +1595,7 @@ ValueError: 7
 >>> f().throw("abc")     # throw on just-opened generator
 Traceback (most recent call last):
   ...
-TypeError: exceptions must be classes, or instances, not str
-
+abc
 
 Now let's try closing a generator:
 

Objects/genobject.c

@@ -249,7 +249,10 @@ gen_throw(PyGenObject *gen, PyObject *args)
 			Py_INCREF(typ);
 		}
 	}
-	else {
+
+	/* Allow raising builtin string exceptions */
+
+	else if (!PyString_CheckExact(typ)) {
 		/* Not something you can raise.  throw() fails. */
 		PyErr_Format(PyExc_TypeError,
 			     "exceptions must be classes, or instances, not %s",