sentinel: broken weakref on destruction

[jasonk]

Pull Request check-list

Please make sure to review and check all of these items:

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

Description of change

This is basically just a copy of the changes that were made in #2771 to fix the problem reported in #2767, which only fixed the sync Sentinel. This is just the same changes applied to the async Sentinel.