Fix bundle() on Python 3.13 by snapshotting the caller's locals

[Max Freedom Pollard (MaxFreedomPollard)]

Fixes #43.

Root cause

bundle() captures the calling frame's locals so a trainable operator can be re-executed with them:

prev_f_locals = inspect.stack()[1].frame.f_locals

Python 3.13 implemented PEP 667, which changed frame.f_locals to return a live FrameLocalsProxy instead of a dict. FunModule.__init__ then rejects it:

AssertionError: _ldict must be a dictionary. or None

Every bundle() call fails on 3.13 as a result, so the package is effectively unusable there — which is what #43 reports. Ching-An Cheng (@chinganc) identified this cause in the issue thread; this PR is the fix for it.

The fix

prev_f_locals = dict(inspect.stack()[1].frame.f_locals)

I went with a snapshot at the capture site rather than relaxing the isinstance(_ldict, dict) check, for two reasons:

• It restores the pre-3.13 behaviour exactly. Before 3.13, f_locals handed back a point-in-time dict copy. FrameLocalsProxy is a live view of the frame, so merely accepting it would leave 3.13 with subtly different semantics from every other version.
• Nothing else has to change. FunModule still receives a real dict, so the existing assertion and the _ldict.copy() on the next line keep working untouched.

FrameLocalsProxy is a Mapping, so dict(...) is the documented way to materialise it, and on ≤3.12 this is just a copy of a dict that was about to be copied anyway.

I also checked the other frame manipulation in bundle.py for 3.13 breakage — update_local() calls ctypes.pythonapi.PyFrame_LocalsToFast, which I expected to be gone in 3.13. It is still exported and still works, and the overwrite_python_recursion path it serves passes on 3.13, so nothing further is needed.

Coverage

tests/unit_tests/test_bundle.py already exercised the broken path — it applies @bundle() inside a function, which is exactly what triggers the capture — it just never ran on 3.13. So this PR:

• adds an explicit assertion that the captured locals are a dict containing the caller's variables, which pins the behaviour rather than testing it incidentally
• extends the CI matrix to ["3.9", "3.13"] with fail-fast: false, so 3.13 is actually covered from now on

Without the fix, test_bundle.py fails on 3.13 with the AssertionError above and passes on 3.9. With it, both pass.

Verification

	3.9	3.12	3.13
python tests/unit_tests/run.py	22/22, exit 0	22/22, exit 0	22/22, exit 0

The matrixed workflow, run on this exact commit in my fork: https://github.com/MaxFreedomPollard/Trace/actions/runs/33941552555 — both legs green, 22/22 each.

ruff==0.0.261 reports the same 60 pre-existing findings before and after this change, and black==23.3.0 proposes no changes to any added line (both files already differ from black on main, so I left the rest alone). No API, signature, or dependency changes.