gh-96092: Fix traceback.walk_stack(None) skipping too many frames (#129330)

As it says in its documentation, walk_stack was meant to just
follow `f.f_back` like other functions in the traceback module.
Instead it was previously doing `f.f_back.f_back` and then this
changed to `f_back.f_back.f_back.f_back' in Python 3.11 breaking
its behavior for external users.

This happened because the walk_stack function never really had
any good direct tests and its only consumer in the traceback module was
`extract_stack` which passed the result into `StackSummary.extract`.
As a generator, it was previously capturing the state of the stack
when it was first iterated over, rather than the stack when `walk_stack`
was called. Meaning when called inside the two method deep
`extract` and `extract_stack` calls, two `f_back`s were needed.
When 3.11 modified the sequence of calls in `extract`, two more
`f_back`s were needed to make the tests happy.

This changes the generator to capture the stack when `walk_stack` is
called, rather than when it is first iterated over. Since this is
technically a breaking change in behavior, there is a versionchanged
to the documentation. In practice, this is unlikely to break anyone,
you would have been needing to store the result of `walk_stack` and
expecting it to change.

Lib/test/test_traceback.py

@@ -3229,11 +3229,17 @@ class TestStack(unittest.TestCase):
     def test_walk_stack(self):
         def deeper():
             return list(traceback.walk_stack(None))
-        s1 = list(traceback.walk_stack(None))
-        s2 = deeper()
+        s1, s2 = list(traceback.walk_stack(None)), deeper()
         self.assertEqual(len(s2) - len(s1), 1)
         self.assertEqual(s2[1:], s1)
 
+    def test_walk_innermost_frame(self):
+        def inner():
+            return list(traceback.walk_stack(None))
+        frames = inner()
+        innermost_frame, _ = frames[0]
+        self.assertEqual(innermost_frame.f_code.co_name, "inner")
+
     def test_walk_tb(self):
         try:
             1/0