Fix ValueError on flow.replace

[sp1thas]

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• adds new tests (if appropriate)
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What does this PR change?

Fixes flow.replace method. During the action of flow.replace updates the flow.slugs dictionary and removes the old task.

Why is this PR important?

Fixes bug #2919

[marvin-robot]

Here I am, brain the size of a planet and they ask me to welcome you to Prefect.

So, welcome to the community @sp1thas! 🎉 🎉

[codecov]

Codecov Report

Merging #2924 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

[jcrist]

Thanks @sp1thas, this looks good to me.