Update ci/cd to work with IIIFDownloader

[rwood-97]

Summary

Since we are currently using piffle from my dev branch, we can't add it as a dependency.
So instead, for now, I've added that we explicitly install it in our CI and now our tests should hopefully work.

I can then push a version update to pypi.

Checklist before assigning a reviewer (update as needed)

• Self-review code
• Ensure submission passes current tests
• Add tests
• Update relevant docs
• Update changelog

Reviewer checklist

Please add anything you want reviewers to specifically focus/comment on.

• Everything looks ok?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	79.30% <100.00%> (+1.40%)	⬆️

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Files with missing lines	Coverage Δ
tests/test_download/test_iiif_downloader.py	92.17% <100.00%> (ø)
tests/test_import.py	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

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