Thanks for working on this.

This is not my area of expertise, but what you describe sounds like an RLock, and there is a C implementation of RLock in Python3.  Could you just use that for Python3?

Also, very minor comments on the patch format (I'm not in a position to review the patch itself): we prefer not to add additional copyright notices (some files have older ones).  My understanding is you have the copyright by virtue of having published the patch here, and your contributor agreement on file allows us to incorporate it into the codebase, and nothing more is needed.

I don't believe we generally include bug fixes in What's New, unless they are significant enough behavior changes that they don't get put into the older versions.  It's Raymond's call, though.