fix(backport): Skip callbacks with dead weakrefs while iterating over callbacks

[matthewfeickert]

Description

• Backport PR fix: Skip callbacks with dead weakrefs while iterating over callbacks #2310
• Check refs while processing callbacks in case a callee is de-ref'd during the callback process.
• Additionally flush after processing callbacks in case any callback de-ref's a callee.
• Note that no additional tests are added for this case as the problem arises from and intermediate callback triggering a garbage-collect. It is unclear how to force this scenario deterministically in testing.
• Add Jonas Rembser to contributors list.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Backport PR https://github.com/scikit-hep/pyhf/pull/2310
* Check refs while processing callbacks in case a callee is de-ref'd during
  the callback process.
* Additionally flush after processing callbacks in case any callback de-ref's
  a callee.
* Note that no additional tests are added for this case as the problem arises from
  and intermediate callback triggering a garbage-collect. It is unclear how to force
  this scenario deterministically in testing.
* Add Jonas Rembser to contributors list.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release/v0.7.x@a8b6fce). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@                Coverage Diff                @@
##             release/v0.7.x    #2321   +/-   ##
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  Coverage                  ?   98.21%           
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  Files                     ?       69           
  Lines                     ?     4542           
  Branches                  ?      803           
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  Hits                      ?     4461           
  Misses                    ?       47           
  Partials                  ?       34           

Flag	Coverage Δ
contrib	27.52% <0.00%> (?)
doctest	60.98% <0.00%> (?)
unittests-3.10	96.25% <0.00%> (?)
unittests-3.7	96.23% <0.00%> (?)
unittests-3.8	96.27% <0.00%> (?)
unittests-3.9	96.30% <0.00%> (?)

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