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.

Unit tests pass in kgb, Djblets, and Review Board with all supported

versions of Python.