Avoid during-iteration dictionary changes

[douglasdavis]

• Closes #xxxx
• Tests added / passed
• Passes black distributed / flake8 distributed / isort distributed

cc @jrbourbeau; this locally fixes what popped up in #5257 (comment). Definitely open to discussion on a different style of fix. I'm looking to see how that test causes an update of the dictionary during iteration

[jrbourbeau]

Thanks @douglasdavis! Looking at the diff for #5120 there were several places where we were previously using list(...) or taking a slice for plugin in self.plugins[:] (search for for plugin in when looking at the diff), would you mind making updates in those other places too?

[douglasdavis]

Sure -- went through and adjusted all of the other instances of self.plugins.values() to list(self.plugins.values()). Looks like one worker plugin associated test and an async test are failing. Not sure if they're related to this. (they pass locally for me)

[jrbourbeau]

Thanks @douglasdavis! I'm also not able to reproduce that failure locally. Let's just keep an eye out for it