Testing by changing list(sys.modules.items()) to sys.modules.copy().items() internally with a large integration test that was reliably flaky on this line before shows that the .copy().items() worked.  The test is reliable again.

So I've gone ahead and pushed those changes in.  PyDict_Copy()'s implementation at first ~5 minute glance did not appear to have calls to code I'd expect to re-enter Python releasing the GIL.  But I didn't try to do a deep dive.  It works for us and is logically equivalent.