Fix saving __qualname__ of functions

[dfremont]

Summary

This PR fixes the saving of the __qualname__ attribute of functions when it is the same as __name__ but different from __code__.co_qualname (a situation that arises when using functools.wraps: see issue #602 for details).

Fixes #602. This changes existing behavior, so is technically not backwards-compatible, but I think the existing behavior was clearly wrong. (It was also inconsistent between module-level and local functions.)

Checklist

Documentation and Tests

• Added relevant tests that run with python tests/__main__.py, and pass.

Release Management

• Added "Fixes #NNN" in the PR body, referencing the issue (#NNN) it closes.
• Added a comment to issue #NNN, linking back to this PR.
• Added rationale for any breakage of backwards compatibility.
• Requested a review.

[dfremont]

Whoops, I forgot co_qualname doesn't exist in Python 3.10. Before it was introduced, types.FunctionType used co_name instead, so I've added code to fall back to that.

@mmckerns or @anivegesana (author of the specific line I modified) could you please review this when you have a moment?

[mmckerns]

The unfortunate thing about changing behavior is that we also have to make sure the changes to the code do not impact the ability to unpickle old pickles that were created before the change. I'll review your changes -- can you check your changes versus some old pickles? If it fails, we would need to add a shim to the code to handle old pickles.

[dfremont]

Thanks, Mike. The 1-line change I made is in save_function, which is only invoked during pickling, not unpickling (if I understand correctly). So handling of existing pickles will not change.