Fix a crash when accessing locals on Python 3.13.

Python 3.13 is introducing some behind-the-scenes changes for how local
variables are accessed within a function, with an eye toward performance
and reliability. This normally wouldn't impact users in any way, but it
does impact kgb, given the work required to dynamically patch functions.

The problem was that when spying on closures where the inner function
references an argument from a parent function, any attempt to access
`locals()` would fail. Since we need to store `locals()` in order to
access variables in the function, this broke us.

The reason it happens is a bit unclear, but the fix is not.

Historically, when creating a new `CodeType` for a patched function, we
would retain the `co_freevars` and `co_cellvars` of the old function,
which was at some point necessary for closures. Best I can tell, this
was needed when creating a new merged `CodeType` on these older
releases, but does not seem to be important anymore (at least when using
`CodeType.replace()`).

Removing the overridden `co_freevars` and `co_cellvars` and using the
new values only ended up fixing the `locals()` access. There doesn't
seem to be a need to override these. I've tested with running the kgb,
Review Board, Djblets, and RBTools test suites for all affected Python
versions (3.8 through 3.13) without issue.

Testing Done:
Unit tests pass in kgb, Djblets, and Review Board with all supported
versions of Python.

Reviewed at https://reviews.reviewboard.org/r/14168/

kgb/spies.py

@@ -993,10 +993,14 @@ def _build_spy_code(self, func, forwarding_call):
             # one when possible. On Python 3.11, this will ensure that
             # state needed for exceptions (co_positions()) will be set
             # correctly.
+            #
+            # NOTE: Prior to kgb 7.2, we had set co_freevars and co_vellvars
+            #       here. This caused crashes with Python 3.13 beta 2 (the
+            #       latest release as of this writing -- June 19, 2024). We
+            #       don't appear to actually need or want to set these on
+            #       Python 3, so we removed this.
             replace_kwargs = {
                 'co_name': old_code.co_name,
-                'co_freevars': old_code.co_freevars,
-                'co_cellvars': old_code.co_cellvars,
             }
 
             if pyver >= (3, 11):

kgb/tests/test_function_spy.py

@@ -1188,6 +1188,24 @@ def inline_func():
         self.assertTrue(func.called)
         self.assertEqual(d, {'called': True})
 
+    def test_call_with_function_providing_closure_access_args(self):
+        """Testing FunctionSpy calls with an inline function accessing
+        parent's arguments
+        """
+        # NOTE: This originally caused a crash on Python 3.13 beta 2.
+        #       See: https://github.com/beanbaginc/kgb/issues/11
+        def func(arg):
+            def inline_func():
+                print(arg)
+
+            return 123
+
+        self.agency.spy_on(func)
+
+        d = func(42)
+        self.assertEqual(d, 123)
+        self.assertTrue(func.called)
+
     def test_call_with_bound_method_with_list_comprehension_and_self(self):
         """Testing FunctionSpy calls for bound method using a list
         comprehension referencing 'self'

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{27,36,37,38,39,310,311},pypy{37,38}
+envlist = py{27,36,37,38,39,310,311,312,313},pypy{37,38}
 skipsdist = True
 
 [testenv]