Fix for an obscure bug introduced by revs 46806 and 46808, with a test.

The problem of checking too eagerly for recursive calls is the
following: if a RuntimeError is caused by recursion, and if code needs
to normalize it immediately (as in the 2nd test), then
PyErr_NormalizeException() needs a call to the RuntimeError class to
instantiate it, and this hits the recursion limit again...  causing
PyErr_NormalizeException() to never finish.

Moved this particular recursion check to slot_tp_call(), which is not
involved in instantiating built-in exceptions.

Backport candidate.

Lib/test/test_exceptions.py

@@ -305,6 +305,18 @@ class ExceptionTests(unittest.TestCase):
         x = DerivedException(fancy_arg=42)
         self.assertEquals(x.fancy_arg, 42)
 
+    def testInfiniteRecursion(self):
+        def f():
+            return f()
+        self.assertRaises(RuntimeError, f)
+
+        def g():
+            try:
+                return g()
+            except ValueError:
+                return -1
+        self.assertRaises(RuntimeError, g)
+
 def test_main():
     run_unittest(ExceptionTests)